Objective lens, optical pickup apparatus, and optical information recording reproducing apparatus

ABSTRACT

An objective lens relating to the present invention includes a first optical path difference providing structure in which a first basic structure and a second basic structure are overlapped with each other. The first basic structure is a blaze-type structure which emits a Xth-order diffracted light flux, when the first light flux passes through the first basic structure, where the value of X is an odd integer. At least a part of the first basic structure arranged around an optical axis includes a step facing an opposite direction to the optical axis. The second basic structure is a blaze-type structure which emits a Lth-order diffracted light flux, when the first light flux passes through the second basic structure, where the value of L is an even integer. At least a part of the second basic structure arranged around the optical axis includes a step facing the optical axis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/773,588, which was filed with the U.S. Patent and Trademark Office onMay 4, 2010. Priority is claimed on Japanese Patent Application Nos.2009-112918 filed on May 7, 2009, 2009-112919 filed on May 7, 2009,2009-148535 filed on Jun. 23, 2009, and 2010-050599 filed on Mar. 8,2010, in Japanese Patent Office, the entire contents of which are herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to an optical pickup apparatus whichcompatibly records and/or reproduces (which may be described as“records/reproduces” in the present specification) information fordifferent types of optical discs, and further relates to an objectivelens and an optical information recording reproducing apparatus.

BACKGROUND ART

There has been a trend in recent years for a laser light source which isused as alight source in an optical pickup apparatus for reproducinginformation which has been recorded in an optical disc and for recordinginformation on an optical disc, toward a shorter wavelength. Forexample, laser light sources with 390-420 nm wavelength, such as ablue-violet semiconductor laser, are reaching the stage of practicalapplication. By using these blue-violet laser light sources, informationof 15-20 GB can be recorded on an optical disc with a diameter of 12 cmby using an objective lens with the same numerical aperture (NA) as thatfor DVD (Digital Versatile Disc), and information of 23-25 GB can berecorded onto an optical disc with a diameter of 12 cm by using anobjective lens with increased NA up to 0.85.

As an example of an optical disc using the above-described objectivelens with NA 0.85, there is cited a BD (Blu-ray Disc). Since increasedcomma is generated because of a tilt (skew) of the disc, a BD has beendesigned so that a protective layer has thinner thickness (which is 0.1mm, while that of DVD is 0.6 mm) than that of DVD, to reduce the amountof comma caused by the skew.

On the other hand, it is considered that an optical disc player/recorder(optical information recording reproducing apparatus) is worthless as aproduct when the optical disc player/recorder is capable ofrecording/reproducing information just for BDs properly. Taking accountof a fact that, at present, DVDs and CDs (Compact Discs) storing variouskinds of information have been on the market, it is not sufficient thatthe optical disc player/recorder can record/reproduce information justfor BDs, and an attempt providing an optical disc player/recordercapable to record/reproduce information also for DVDs and CDs which havealready been owned by users, leads to enhancement of a commercial valueof the optical disc player/recorder for BDs. From such the background,an optical pickup apparatus installed in the high-density optical discplayer/recorder is required to be capable of appropriatelyrecording/reproducing information not only for BDs but also for a DVDsand a CDs.

As a method by which information can be adequately recorded/reproducedwhile the compatibility is maintained to anyone of BDs, DVDs and CDs,there can be considered a method to selectively switch an optical systemfor BDs and an optical system for DVDs and CDs, corresponding to arecording density of an optical disc on which information isrecorded/reproduced. However, it is disadvantageous for thesize-reduction and increases a cost, because plural of optical systemsare needed.

Accordingly, in order to simplify a structure of the optical pickupapparatus and to intend a reduction of its cost, it is preferable toform the optical system for BDs and the optical system for DVDs and CDsinto a common optical system, to reduce the number of optical partsforming the optical pickup apparatus as much as possible, even in theoptical pickup apparatus with compatibility. Then, providing the commonobjective lens which is arranged with facing an optical disc, is mostadvantageous for the simplification of the structure and for costreduction of the optical pickup apparatus. In order to obtain a commonobjective lens for plural kinds of optical discs which use differentwavelengths for recording/reproducing information, it is required that adiffractive structure having a wavelength dependency for the sphericalaberration, is formed in the objective optical system.

JP-A No. 2008-293630 has disclosed an objective lens which includes astructure in which two basic structures each provided as a diffractivestructure are overlapped with each other and can be commonly used forthree types of optical discs, and also has disclosed an optical pickupapparatus in which this objective optical system is mounted.

Two of the three types of optical disc disclosed in JP-A No. 2008-293630are DVDs and CDs. With respect to the rest, JP-A No. 2008-293630 hasjust touched about BD, and all of its examples actually employ HD-DVD,which suggests that JP-A No. 2008-293630 has been provided withemphasizing a compatibility of three types of optical disc of DVD, CDand HD-DVD rather than BD. The inventor has studied an applicability ofthe examples of the application to achieving compatibility of threetypes of optical disc of BDs, DVDs, and CDs, and has found an issue thata working distance of the objective lens for CD becomes so small thatthe objective lens can touch an optical disc during recording andreproducing operations. Herein, a working distance means a distancealong the optical axis from a surface of the optical disc to a surfacevertex of the optical surface of the objective lens facing the opticaldiscs.

SUMMARY

The present invention is achieved with considering the above problem,and provides an optical pickup apparatus, an optical informationrecording reproducing apparatus and an objective lens suitable to them,which realize that information can be recorded and/or reproducedcompatibly for three kinds of optical disc of BDs, DVDs and CDs by usinga common objective lens, and that a sufficient working distance can besecured even for CDs which has a thick substrate.

An objective lens as an embodiment of the invention is provided for usein an optical pickup apparatus which comprises a first light source foremitting a first light flux having a first wavelength λ1, a second lightsource for emitting a second light flux having a second wavelength λ2(λ2>λ1), a third light source for emitting a third light flux having athird wavelength λ3 (λ3>λ2). The optical pickup apparatus records and/orreproduces information with the first light flux on an informationrecording surface of a first optical disc having a protective substratewith a thickness t1, records and/or reproduces information with thesecond light flux on an information recording surface of a secondoptical disc having a protective substrate with a thickness t2 (t1<t2),and records and/or reproduces information with the third light flux onan information recording surface of a third optical disc having aprotective substrate with a thickness t3 (t2<t3). The objective lenscomprises: an optical surface including a central area, an intermediatearea surrounding the central area, and a peripheral area surrounding theintermediate area. The central area comprises a first optical pathdifference providing structure and the intermediate area comprises asecond optical path difference providing structure. The objective lensconverges the first light flux which passes through the central area,onto the information recording surface of the first optical disc so thatinformation can be recorded and/or reproduced on the informationrecording surface of the first optical disc. The objective lensconverges the second light flux which passes through the central area,onto the information recording surface of the second optical disc sothat information can be recorded and/or reproduced on the informationrecording surface of the second optical disc. The objective lensconverges the third light flux which passes through the central area,onto the information recording surface of the third optical disc so thatinformation can be recorded and/or reproduced on the informationrecording surface of the third optical disc. The objective lensconverges the first light flux which passes through the intermediatearea, onto the information recording surface of the first optical discso that information can be recorded and/or reproduced information on theinformation recording surface of the first optical disc. The objectivelens converges the second light flux which passes through theintermediate area, onto the information recording surface of the secondoptical disc so that information can be record and/or reproduceinformation on the information recording surface of the second opticaldisc. The objective lens does not converge the third light flux whichpasses through the intermediate area, onto the information recordingsurface of the third optical disc so that information can be recordedand/or reproduced on the information recording surface of the thirdoptical disc. The objective lens converges the first light flux whichpasses through the peripheral area, onto the information recordingsurface of the first optical disc so that information can be recordedand/or reproduced on the information recording surface of the firstoptical disc. The objective lens does not converge the second light fluxwhich passes through the peripheral area, onto the information recordingsurface of the second optical disc so that information can be recordedand/or reproduced on the information recording surface of the secondoptical disc. The objective lens does not converge the third light fluxwhich passes through the peripheral area, onto the information recordingsurface of the third optical disc so that information can be recordedand/or reproduced on the information recording surface of the thirdoptical disc. The first optical path difference providing structurecomprises a first basic structure and a second basic structure which areoverlapped with each other. The first basic structure is a blaze-typestructure, which emits a Xth-order diffracted light flux with a largerlight amount than diffracted light fluxes with any other diffractionorder, when the first light flux passes through the first basicstructure, which emits a Yth-order diffracted light flux with a largerlight amount than diffracted light fluxes with any other diffractionorder, when the second light flux passes through the first basicstructure, and which emits a Zth-order diffracted light flux with alarger light amount than diffracted light fluxes with any otherdiffraction order, when the third light flux passes through the firstbasic structure, where a value of X is an odd integer. At least apart ofthe first basic structure arranged around an optical axis in the centralarea comprises a step facing an opposite direction to the optical axis.The second basic structure is a blaze-type structure which emits aLth-order diffracted light flux with a larger light amount thandiffracted light fluxes with any other diffraction order, when the firstlight flux passes through the second basic structure, which emits aMth-order diffracted light flux with a larger light amount thandiffracted light fluxes with any other diffraction order, when thesecond light flux passes through the second basic structure, and whichemits a Nth-order diffracted light flux with a larger light amount thandiffracted light fluxes with any other diffraction order, when the thirdlight flux passes through the second basic structure, where a value of Lis an even integer. At least apart of the second basic structurearranged around the optical axis in the central area comprises a stepfacing the optical axis.

These and other objects, features and advantages according to thepresent invention will become more apparent upon reading of thefollowing detailed description along with the accompanied drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings which are meant to be exemplary,not limiting, and wherein like elements numbered alike in severalFigures, in which:

FIG. 1 is a diagram showing objective lens OL according to the presentembodiment, which is a single lens and is viewed along the optical axis;

FIG. 2 is a diagram showing how a third light flux which has passedthrough the objective lens forms a spot on an information recordingsurface of a third optical disc;

FIGS. 3 a-3 d are sectional views taken along the optical axis, showingan example of an optical path difference providing structure;

FIG. 4 a is a diagram showing steps facing the optical axis and FIG. 4 bis a diagram showing steps facing the opposite direction to the opticalaxis;

FIG. 5 a is a diagram showing a form such that steps faces the opticalaxis around the optical axis, the directions of steps are switchedmidway, and steps face the opposite direction to the optical axis aroundthe intermediate area, and FIG. 5 b is a diagram showing a form suchthat steps faces the opposite direction to the optical axis around theoptical axis, the directions of steps are switched midway, and stepsface the optical axis around the intermediate area;

FIG. 6 is a schematic diagram of the first optical path differenceproviding structure;

FIG. 7 is a diagram showing an outline of optical pickup apparatus PU1of the present embodiment which can record and/or reproduce informationproperly for BDs, DVDs and CDs provided as different types of opticaldisc;

FIGS. 8 a-8 c are spherical aberration diagrams of Example 1;

FIGS. 9 a-9 c are diagrams showing a wavelength dependency ofdiffraction efficiency of Example 1; and

FIG. 10 is a sectional view schematically showing the first optical pathdifference providing structure, the second optical path differenceproviding structure, and the third optical path difference providingstructure which are formed on a flat element.

DESCRIPTION OF EMBODIMENTS

Hereafter, although embodiments of the present invention will bedescribed in details, the present invention is not limited to thisembodiment.

The inventor of the prevent invention, as a result of earnest andextensive study, has found the following problem. In all the embodimentsdisclosed in JP-A No. 2008-293630, steps of a basic structure whichexhibits third diffraction order for a blue-violet light flux, face thedirection of the optical axis. Because of such the structure, in a thickobjective lens which is used for achieving compatibility of three typesof optical disc of BDs, DVDs, and CDs and is thick along the opticalaxis, its working distance becomes short when the objective lens worksfor CDs.

Based on the above viewpoint, the inventor has found that, even in athick objective lens which is used for achieving compatibility of threetypes of optical disc of BDs, DVDs, and CDs and is thick along theoptical axis, a sufficient working distance can be secured when theobjective lens works for CDs, by providing steps of a basic structurewhich exhibits an odd diffraction order for a blue-violet light flux andby arranging the step to face the opposite direction to the opticalaxis.

It is preferable that another basic structure is overlapped with (issuperimposed to) the above basic structure in order to achieve thecompatibility of the three types of optical disc of BDs, DVDs, and CDs.Accordingly, the inventor has provided a structure by overlapping twotypes of steps together as follows. First, there is provided steps of abasic structure which diffracts a diffracted light flux with an odddiffraction order for a blue-violet light flux, and the steps in thebasic structure are positioned to face the opposite direction to theoptical axis. Second, there is provided steps of another basic structurewhich diffracts a diffracted light with an even diffraction order for ablue-violet light flux, and the steps of another basic structure arepositioned to have the direction of the optical axis. The inventor hasfound that, by overlapping these steps together, the height of stepsmeasured after the steps are overlapped together can be controlled notto be excessive high with achieving the compatibility of the three typesof optical disc of BDs, DVDs, and CDs. Accordingly, it enables torestrict loss of light amount caused due to manufacturing error andenables to restrict a fluctuation of a diffraction efficiency causedwhen a wavelength changes. By these effects, information can be recordedand/or reproduced for three types of optical discs of BDs, DVDs, and CDssatisfactory by using a common objective lens.

Further, the above structure can provide an objective lens withwell-balanced light utilizing efficiency so as to maintain high lightutilizing efficiency for each of the three types of optical disc of BDs,DVDs, and CDs. Since the objective lens employs a first optical pathdifference providing structure formed by overlapping two types ofstructure as the first basic structure and the second basic structuretogether, design flexibility of the objective lens can be secured morelargely compared with an optical path difference providing structureformed by only one basic structure such as a staircase structure, butnot overlapping plural basic structures. Such design flexibility isadvantageous especially in a lens with a small effective diameter.

Furthermore, by the above features, an aberration caused when awavelength of an incident light changed to be longer, can be changedtoward under-corrected (deficient correction) direction. Thereby, anaberration caused when a temperature of an optical pickup apparatusraises can be controlled to be small, which enables to provide anobjective lens capable of maintaining a stable performance even under atemperature change, when plastic is employed for the material of theobjective lens.

In the objective lens, it is preferable that the value of L is an evennumber whose absolute value is 4 or less, and the value of X is an oddnumber whose absolute value is 5 or less.

The objective lens preferably satisfies (X, Y, Z)=(−1, −1, −1) and (L,M, N)=(2, 1, 1).

In the objective lens, all steps of the first basic structure arrangedin the central area can face the opposite direction to the optical axis.

In the objective lens, a part of the first basic structure arrangedaround the intermediate area in the central area can comprise a stepfacing the optical axis.

In the objective lens, all steps of the second basic structure arrangedin the central area can face the optical axis.

In the objective lens, a part of the second basic structure arrangedaround the intermediate area in the central area can comprise a stepfacing the opposite direction to the optical axis.

In the objective lens, at least apart of the first optical pathdifference providing structure arranged around the optical axis in thecentral area includes both of a step facing an opposite direction to theoptical axis and a step facing the optical axis, and the step facing anopposite direction to the optical axis and the step facing the opticalaxis satisfy the following expressions (1) and (2), where d11 is anamount of the step facing the opposite direction to the optical axis,d12 is an amount of the step facing the optical axis, and n is arefractive index of the objective lens at the wavelength0.6·(λ1/(n−1))<d11<1.5·(λ1/(n−1)),  (1)0.6·(λ1/(n−1))<d12<1.5·(2λ1/(n−1)).  (2)

These embodiments enable to record and/or reproduce informationcompatibly for the three types of optical discs of BDs, DVDs, and CDswith a common objective lens. Further, these embodiments providereduced-height steps arranged on the objective lens, which provides anobjective lens with small manufacturing error, a small loss in lightamount, and stable diffraction efficiency even when a wavelength of anincident light changes. Further, an objective lens with well-balancedlight utilizing efficiency so as to maintain high light utilizingefficiency for each of the three types of optical disc of BDs, DVDs, andCDs, can be provided.

Additionally, those embodiments do not exhibit wavelength characteristicwhich is excessively under-corrected or is excessively over-corrected,but exhibit wavelength characteristic which is under-corrected to aproper degree. Thereby, an aberration caused when a temperature of anoptical pickup apparatus raises can be controlled to be small, whichenables to provide an objective lens capable of maintaining a stableperformance even under a temperature change.

In the above objective lens, the step facing an opposite direction tothe optical axis and the step facing the optical axis preferably satisfythe expressions (1) and (2) in all over the central area.

It is more preferable that the step facing an opposite direction to theoptical axis and the step facing the optical axis satisfy the followingexpressions (1′) and (2′):0.9·(λ1/(n−1))<d11<1.5·(λ1/(n−1)),  (1′)0.9·(λ1/(n−1))<d12<1.5·(λ1/(n−1)).  (2′)

In the above objective lens, the step facing an opposite direction tothe optical axis and the step facing the optical axis preferably satisfythe expressions (1′) and (2′) in all over the central area.

In the objective lens, it is preferable that the number of steps facingan opposite direction to the optical axis is more than the number ofsteps facing the optical axis, in the central area.

When information is recorded and/or reproduced for the third opticaldisc, the third light flux entering the first optical path differenceproviding structure is emitted mainly as diffracted light which isnecessary and is used for recording and/or reproducing information, andpartially as diffracted light which is unwanted and is not used forrecording and/or reproducing information. By making a pitch of the firstoptical path difference providing structure small, or by preferablyincreasing the number of steps facing the opposite direction to theoptical axis to makes the pitch small, a light converging position ofthe unwanted diffracted light can be located away from a lightconverging position of the necessary diffracted light. Thereby, adetection error caused by unwanted diffracted light converged onalight-receiving element, can be avoided, which is preferable. Further,such the structure also preferable in a way that a working distance forCDs is more easily ensured satisfactory.

In the objective lens, when any one of the first light flux, the secondlight flux and the third light flux whose wavelength is longer than thecorresponding wavelength among the first wavelength, the secondwavelength, and the third wavelength enters the objective lens, theobjective lens preferably generates a third order spherical aberrationand a fifth order spherical aberration each of which is under-corrected.

By employing the structure achieving the above, an aberration causedwhen a temperature in an optical pickup apparatus raises, can bereduced. It enables to provide an objective lens capable of maintaininga stable performance even under the temperature changes, when theobjective lens is made of plastic.

The objective lens preferably satisfies the following expression.1.0≦d/f≦1.5  (3)

In the expression, d is a thickness (mm) of the objective lens along theoptical axis and f is a focal length (mm) of the objective lens for thefirst light flux.

When an objective lens suitable to an optical disc with high NA using ashort wavelength, such as BDs, is provided, astigmatism and decentrationcoma can be easily caused in the objective lens. However, the abovestructure can control a generation of the astigmatism and decentrationcoma to be small.

When the objective lens satisfies the expression (3), the objective lensbecomes a thick objective lens whose thickness along the optical axis islarge, thereby, a working distance for recording and/or reproducinginformation for CD, tends to be short. However, the objective lens ofthe embodiment provides the first optical path providing structure tosufficiently secure the working distance. Therefore, the effects of thepresent invention becomes significantly.

The objective lens preferably generates a longitudinal chromaticaberration of 0.9 μm/nm or less.

The inventor, as a result of earnest and extensive study, has come toconsider to strengthen a negative power in a basic structure, in orderto secure a larger working distance for CD. In such case, by making apitch of a structure generating a diffracted light with an odddiffraction order when the objective lens works for BD, to be small, thenegative power of the basic structure can be strengthened. However, theinventor has also found that a trade-off characteristics that alongitudinal chromatic aberration becomes relatively large in place ofthe strengthened negative power. On the other hand, some semiconductorlasers can have a broadened wavelength spectrum (namely, have awavelength band which extends toward upward and downward from thereference wavelength) because of superposition of high frequencycurrent. The inventor has found a problem that when alight flux whosewavelength spectrum is broadened enters an objective lens in whichlongitudinal chromatic aberration is relatively large, a side-lobe in aprofile of a converged spot increases and cross talk is easily caused.

Therefore, by forming a first optical path difference providingstructure, especially the first basic structure, with a diffractionpitch such that the longitudinal chromatic aberration is controlled tobe 0.9 μm/nm or less, generation of the cross talk can be effectivelycontrolled even when a semiconductor laser with broadened wavelengthspectrum because of superposition of high frequency current, isemployed.

In the objective lens, it is preferable that the longitudinal chromaticaberration is 0.4 μm/nm or more.

According to the above embodiment, the diffraction pitch of the firstbasic structure does not become excessively small, and a sufficientworking distance for CDs can be secured.

The objective lens preferably satisfies the following expression.0.002≦p/fl≦0.004  (23)

In the expression, p is a minimum pitch in the first optical pathdifference providing structure in the central area and f1 is a focallength of the objective lens for at the first wavelength.

By satisfying the above expression, the objective lens can provide arestricted longitudinal chromatic aberration with securing asatisfactory working distance for CD, and can effectively control ageneration of cross talk even when a semiconductor laser with abroadened wavelength spectrum because of superposition of high frequencycurrent, is employed. Further, a converging position of the unwanteddiffracted light can be located away from a converging position of thenecessary diffracted light and it avoids a detection error causedbecause the unwanted light is converged onto a light-receiving element.

In the objective lens, it is preferable that the first optical pathdifference providing structure in the central area has a minimum pitchof 15 μm or less.

When information is recorded and/or reproduced for the third opticaldisc, the third light flux entering the first optical path differenceproviding structure is emitted mainly as diffracted light which isnecessary and is used for recording and/or reproducing information, andpartially as diffracted light which is unwanted and is not used forrecording and/or reproducing information. By making a pitch of the firstoptical path difference providing structure small, or by preferablymaking a pitch of the first basic structure small, a light convergingposition of the unwanted diffracted light can be located away from alight converging position of the necessary diffracted light. Thereby, adetection error caused by unwanted diffracted light converged on alight-receiving element, can be avoided, which is preferable.

The objective lens preferably satisfies the following expressions (4),(5), and (6).−0.01<m1<0.01  (4)−0.01<m2<0.01  (5)−0.01<m3<0.01  (6)

In these expressions, m1 is a magnification of the objective lens whenthe first light flux enters the objective lens, m2 is a magnification ofthe objective lens when the second light flux enters the objective lens,and m3 is a magnification of the objective lens when the third lightflux enters the objective lens.

Another embodiment of the present invention is an optical pickupapparatus comprising: a first light source for emitting a first lightflux having a first wavelength λ1; a second light source for emitting asecond light flux having a second wavelength λ2 (λ2>λ1); a third lightsource for emitting a third light flux having a third wavelength λ3(λ3>λ2); and the above objective lens. The optical pickup apparatusrecords and/or reproduces information with the first light flux on aninformation recording surface of a first optical disc having aprotective substrate with a thickness t1, records and/or reproducesinformation with the second light flux on an information recordingsurface of a second optical disc having a protective substrate with athickness t2 (t1<t2), and records and/or reproduces information with thethird light flux on an information recording surface of a third opticaldisc having a protective substrate with a thickness t3 (t2<t3).

Another embodiment of the present invention is an optical informationrecording and reproducing apparatus comprising the above optical pickupapparatus.

An optical pickup apparatus relating to the present invention, comprisesat least three light sources: a first light source, a second lightsource, and a third light source. Further, the optical pickup apparatuscomprises alight-converging optical system for converging a first lightflux onto an information recording surface of a first optical disc,converging a second light flux onto an information recording surface ofa second optical disc, and converging a third light flux onto aninformation recording surface of a third optical disc. The opticalpickup apparatus further comprises an light-receiving element forreceiving a reflection light flux from the information recording surfaceof each of the first optical disc, second optical disc, and thirdoptical disc.

The first optical disc comprises a protective substrate with a thicknessof t1 and an information recording surface. The second optical disccomprises a protective substrate with a thickness of t2 (t1<t2) and aninformation recording surface. The third optical disc comprises aprotective substrate with a thickness of t3 (t2<t3) and an informationrecording surface. It is preferable that the first optical discrepresents a BD, the second optical disc represents a DVD, and the thirdoptical disc represents a CD, but optical discs are not limited tothose. Each of the first optical disc, the second optical disc, and thethird optical disc may be a multilayer optical disc with a plurality ofinformation recording surfaces.

In the present specification, BD represents a generic name of opticaldiscs belonging to BD group having a protective substrate with athickness in the range of about 0.05 to 0.125 mm, for which informationis recorded/reproduced with a light flux with a wavelength of about 390to 415 nm by an objective lens with NA of about 0.8 to 0.9. BDs includesuch a disc including only a single information recording layer and sucha disc including two or more information recording layers. Further, DVDin the present specification represents a generic name of optical discsbelonging to DVD group with a protective substrate of about 0.6 mm forwhich information is recorded/reproduced by an objective lens with NA inthe range of about 0.60 to 0.67. DVDs include DVD-ROM, DVD-Video,DVD-Audio, DVD-RAM, DVD-R, DVD-RW, DVD+R and DVD+RW. In the presentspecification, CD represents a generic name of optical discs belongingto CD group having a protective substrate of about 12 mm, for whichinformation is recorded and/or reproduced by an objective lens with NAin the range of about 0.45 to 0.51. CDs include CD-ROM, CD-Audio,CD-Video, CD-R and CD-RW. Among these optical discs, a high densityoptical disc provides the highest recording density. DVD and CD providethe second highest recording density, the third highest recordingdensity, respectively.

Thicknesses t1, t2, and t3 of the protective substrates preferablysatisfy the following conditional expressions (7), (8), and (9), but thethicknesses are not limited to those. Herein, a thickness of aprotective substrate means a thickness of a protective substrate formedon a top of an optical disc. Namely, it means a thickness of aprotective substrate measured from a top of an optical disc to aninformation recording surface placed at the closest position to the top.0.050 mm≦t1≦0.125 mm  (7)0.5 mm≦t2≦0.7 mm  (8)1.0 mm≦t3≦1.3 mm  (9)

In the present specification, each of the first light source, the secondlight source, and the third light source is preferably a laser lightsource. A semiconductor laser, and a silicon laser are preferably usedfor the laser light source. First wavelength λ1 of a first light fluxemitted from the first light source, second wavelength λ2 (λ2>λ1) of asecond light flux emitted from the second light source, third wavelengthλ3 (λ3>λ2) of a third light flux emitted from the third light source,are preferable to satisfy the following expressions (10) and (11).1.5·λ1<λ2<1.7·λ1  (10)1.8·λ1<λ3<2.0·λ1  (11)

When a BD is employed as the first optical disc, the wavelength λ1 ofthe first light source is preferably from 350 nm or more, and 440 nm orless, and more preferably from 390 nm or more, and 415 nm or less. Whena DVD is employed as the second optical disc, the second wavelength λ2of the second light source is preferably from 570 nm or more, and 680 nmor less, and is more preferably from 630 nm or more, and 670 nm or less.When a CD is employed for the third optical disc, the third wavelengthλ3 of the third light source is preferably from 750 nm or more, and 880nm or less, and is more preferably from 760 nm or more, and 820 nm orless.

When a laser light source in which superposition of high frequencycurrent is carried out is employed, it can entail a risk that cross talkis caused. However, by controlling longitudinal chromatic aberration tobe 0.9 μm/nm or less, such the laser light source in which superpositionof high frequency current is carried out can be preferably employed,because a generation of cross talk is restricted. Further, when a laserlight source emitting a light flux with a wavelength spectrum whose fullwidth at the half maximum (a full width of a wavelength spectrum at ahalf of the maximum value of the spectrum) is 0.5 nm or more, isemployed, it can entails larger problem such as cross talk. However, bycontrolling longitudinal chromatic aberration to be 0.9 μm/nm or less,such the laser light source can be preferably employed, because ageneration of cross talk is restricted.

Further, at least two light sources of the first light source, thesecond light source, and the third light source may also be unitized.The unitization means fixing and housing, for example, the first lightsource and the second light source into one package. Additionally to thelight sources, a light-receiving element described below can beunitized.

As a light-receiving element, a photodetector such as a photodiode ispreferably used Light reflected on an information recording surface ofan optical disc enters the light-receiving element, and signal outputtedfrom the light-receiving element is used for obtaining the read signalof the information recorded in each optical disc. Further, change in thelight amount of the spot on the light-receiving element caused becauseof the change in the spot shape and the change in the spot position, isdetected to conduct the focus detection and the tracking detection. Theobjective lens can be moved based on these detections for focusing andtracking of the objective lens. The light-receiving element may becomposed of a plurality of photodetectors. The light-receiving elementmay also have a main photodetector and secondary photodetector. Forexample, the light-receiving element can be provided with a mainphotodetector which receives a main light used for recording andreproducing information, and with two secondary photodetectorspositioned on both sides of the main photodetector so as to receivesecondary light for tracking adjustment by the two secondaryphotodetectors. Alternatively, the light receiving-element may beprovided with a plurality of light-receiving elements corresponding torespective light sources.

The light-converging optical system comprises an objective lens. Thelight-converging optical system preferably comprise a coupling lens suchas a collimation lens other than the objective lens. The coupling lensis a single lens or a group of lenses which is arranged between anobjective lens and a light source and changes divergent angle of a lightflux. The collimation lens is one type of coupling lenses, and is a lensconverting an incident light flux into a parallel light flux. In thepresent specification, an objective lens is an optical system which isarranged to face an optical disc in the optical pickup apparatus, andhas the function which converges alight flux emitted from the lightsource onto an information recording surface of an optical disc. Theobjective lens may be formed of a plurality of lenses and/or opticalelements. Alternatively, the objective lens may be a single lens.Preferably, the objective lens is formed of a single lens. A biconcavesingle lens cam preferably employed as the objective lens. The objectivelens may also be a glass lens, a plastic lens or a hybrid lens in whichan optical path difference providing structure is formed on the glasslens out of resin such as photo-curable resin, UV-curable resin, andthermosetting resin. When the objective lens has a plurality of lenses,a combination of a glass lens and a plastic lens can be used for theobjective lens. When the objective lens has a plurality of lenses and/oroptical elements, a combination of: an optical element in flat plateshape having an optical path difference providing structure; and anaspheric surface lens, which may not have a optical path differenceproviding structure, can be used for the objective lens. The objectivelens preferably comprises a refractive surface which is an asphericsurface. Further, in the objective lens, a base surface where theoptical path difference providing structure is provided, is preferablyan aspheric surface.

When the objective lens is a glass lens, a glass material used for theglass lens preferably has a glass transition point Tg of 500° C. orless, or preferably of 400° C. or less. By using the glass materialwhose glass transition point Tg is 500° C. or less, the material can bemolded at a comparatively low temperature. Therefore, the life of moldscan be prolonged. As an example of the glass material whose glasstransition point Tg is low, there are K-PG325 and K-PG375 (both aretrade names) made by SUMITA Optical glass, Inc.

A glass lens has generally larger specific gravity than a resin lens.Therefore, an objective lens made of a glass lens has larger weight andapply a larger burden to an actuator which drives the objective lens.Therefore, when a glass lens is employed for the objective lens, a glassmaterial having small specific gravity is preferably used for theobjective lens. Specifically, the specific gravity is preferably 4.0 orless, and is more preferably 3.0 or less.

Further, one of important physical values when molding a glass lens, isa linear expansion coefficient “a”. Even if a material with glasstransition point Tg of 400° C. or less is selected, its temperaturedifference from a room temperature is still larger than that of plasticmaterials. When a glass material with a large linear expansioncoefficient is molded to form a lens, the lens can easily have crackswhen the temperature is lowered. It is preferable that the linearexpansion coefficient of a glass material is 200 (10⁻⁷/K) or less and ismore preferable that it is 120 (10⁻⁷/K) or less.

When the objective lens is a plastic lens, it is preferable thatalicyclic hydrocarbon polymer such as cyclic olefin resin is employedfor the objective lens. Among the materials, a preferable resin has:refractive index within the range of 1.54 to 1.60 at the temperature 25°C. and wavelength 405 nm, and ratio of refractive index change dN/dT (°C.⁻¹) which is within the range of −20×10⁻⁵ to −5×10⁻⁵ (more preferably,−10×10⁻⁵ to −8×10⁻⁵), where the ratio of refractive index change iscaused due to the temperature change within the temperature range of −5°C. to 70° C. at the wavelength 405 nm. Further, when a plastic lens isemployed for the objective lens, it is preferable that a plastic lens isalso employed for the coupling lens.

Preferable examples of alicyclic hydrocarbon polymer will be describedbelow.

First preferable example is a resin composition comprising blockcopolymer including polymer block [A] containing a repeating unit [1]represented by the following Formula I, and polymer block [B] containingthe repeating unit [1] represented by the Formula I and a repeating unit[2] represented by the following Formula II, and/or a repeating unit [3]represented by the Formula M. The block copolymer satisfies arelationship of a>b, where a is a mol fraction (mol %) of the repeatingunit [1] in the polymer block [A] and b is a mol fraction (mol %) of therepeating unit [1] in the polymer block [B].

In Formula I, R¹ represents a hydrogen atom or an alkyl group having acarbon number of 1-20, R²-R¹² each independently represent a hydrogenatom, an alkyl group having a carbon number of 1-20, hydroxyl group, analkoxy group having a carbon number of 1-20 or a halogen group.

In Formula 2, R¹³ represents a hydrogen atom or a alkyl group having acarbon number of 1-20.

In Formula 3, R¹⁴ and R¹⁵ each independently represent a hydrogen atomor an alkyl group having a carbon number of 1-20.

Second preferable example is a resin composition containing polymer (A)obtained by an addition polymerization at least of α-olefin with 2-20carbon atoms and monomer composition consisting of cyclic olefinrepresented by the following general formula (IV), and containingpolymer (B) obtained by an addition polymerization of α-olefin with 2-20carbon atoms and monomer composition containing cyclic olefinrepresented by following general formula (V).

In the general formula IV, n is 0 or 1, m is 0 or a positive integer,and q is 0 or 1. R¹ to R¹⁸ and R^(a) and R^(b) each independentlyrepresent a hydrogen atom, a halogen atom or a hydrocarbon group. As forR¹⁵-R¹⁸, each may be bonded to another to form a monocyclic orpolycyclic group, and the monocyclic or polycyclic group formed in thismanner may have double bonds. Also an alkylidene group may also beformed with R¹⁵ and R¹⁶ or R¹⁷ and R¹⁸.

In the general formula V, R¹⁹-R²⁶ each independently represent ahydrogen atom, a halogen atom or a hydrocarbon group.

The following additives may be added to the resin material in order toadd an extra property to the resin material.

(Additives)

It is preferable that at least one type of additive selected from thegroup of phenol type stabilizer, hindered amine type stabilizer,phosphor type stabilizer, and sulfur type stabilizer. By properlyselecting and adding these stabilizers, cloudiness caused when thematerial is continuously irradiated with a light flux with a shortwavelength such as 405 nm, and fluctuation of optical property such asfluctuation of refractive index, can be controlled more properly.

For preferable phenol type stabilizer, usually known ones can beemployed. For example, the followings are cited: acrylate compoundsdescribed in JP-A Nos. 63-179953 and 1-168643 such as2-t-butyl-6-(3-t-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl acrylateand 2,4-di-t-amyl-6-(1-(3,5-di-t-amyl-2-hydroxyphenyl)ethyl)phenylacrylate; an alkyl-substituted phenol compound such asoctadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate,2,2′-methylene-bis(4-methyl-6-t-butylphenol),1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene,tetrakis(methylene-3-(3′,5′-di-t-butyl-4′-hydroxyphenylpropionate)methane,namelypentaerythrimethyl-tetrakis(3-(3,5-di-t-butyl-4-hydroxyphenylpropionate)and triethylene glycolbis-(3-(3-t-butyl-4-hydroxy-5-methylphenyl)-propionate; and a triazinegroup-containing phenol compound such as6-(4-hydroxy-3,5-di-t-butylanilino)-2,4-bisoctyl-1,3,5-triazine,4-bisoctylthio-1,3,5-triazine and2-octylthio-4,6-bis-(3,5-di-t-butyl-4-oxyanilino)-1,3,5-triazine.

For preferable hindered amine type stabilizer, the following samples arecited bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate,bis(2,2,6,6-tetramethyl-4-piperidyl)succinate,bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate,bis(N-octoxy-2,2,6,6-tetramethyl-4-piperidyl)sebacate,bis(N-benzyloxy-2,2,6,6-tetrmethyl-4-piperidyl)sebacate,bis(N-cyclohexyloxy-2,2,6,6-tetramethyl-4-piperidyl)sebacate,bis(1,2,2,6,6-pentamethyl-4-piperidyl)-2-(3,5-di-t-butyl-4-hydroxybenzyl)-2-butylmalonate,bis(1-acroyl-2,2,6,6-tetramethyl-4-piperidyl)2,2-bis(3,5-di-t-butyl-4-hydroxybenzyl)-2-butylmalonate,bis(1,2,2,6,6-pentamethyl-4-piperidyl decanedioate,2,2,6,6-tetramethyl-4-piperidyl methacrylate,4-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxy]-1-[2-(3-(3,5-di-t-buty-14hydroxyphenyl)propionyloxy)ethyl]-2,2,6,6-tetramethyl piperidine,2-methyl-2-(2,2,6,6-tetramethyl-4-piperidyl)amino-N-(2,2,6,6-tetramethyl-4-piperidyl)propionamide,tetrakis(2,2,6,6-tetramethyl-4-piperidyl)1,2,3,4-butanetetracarboxylate,tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)1,2,3,4-butanetetracarboxylate.

As for preferable phosphor type stabilizer, ones usually employed in thefield of resin industry can be employed without any limitation. Forexample, the followings are cited monophosphite compounds such astriphenyl phosphate, diphenylisodecyl phosphate, phenylisodecylphosphate, tris(nonylphenyl)phosphate, tris(dinonylphenyl)phosphate,tris(dinonylphenyl)phosphate, tris(2,4-di-t-butylphenyl)phosphate, and10-(3,5-di-t-butyl-4-hydroxybenzyl)-9,10-dihydro-9-oxa-10-Phosphaphenanthrene-10-oxide;and diphosphite compounds such as4,4′-butylidene-bis(3-methyl-6-t-butylphenyl-di-tridecyl phosphate and4,4′-isopropyridene-bis(phenyl-di-alkyl(C₁₂ to C₁₅)phosphate). Amongthem, the monophosphite compounds are preferable andtris(nonylphenyl)phosphate, tris(dinonylphenyl) phosphate andtris(2,4,-di-t-butylphenyl)phosphate are particularly preferable.

As for preferable sulfur type stabilizer, the following examples arecited dilauryl 3,3-thiodipropionate, dimyrystyl-3,3′-thiodipropionate,distearyl-3,3-thiodipropionate, laurystearyl 3,3-dithiopropionate,pentaerythrytol-tetrakis-(β-laurylstearyl-thio-propionate and3,9-bis-(2-dodecylthioethyl)-2,4,8,10-tetrakispiro[5,5]undecane.

The adding amount of each stabilizer is optionally decided within therange in which the object of the invention is not vitiated; it isusually from 0.01 to 2 parts by weight and preferably from 0.01 to 1part by weight to 100 parts by weight of the alicyclic hydrocarbonpolymer.

(Surfactant)

Surfactant is a compound having a hydrophilic group and a hydrophobicgroup in the identical molecule. The surfactant inhibits cloudiness ofresin composition by adjusting the speed of moisture adhesion to theresin surface and of moisture vaporization from the foregoing surface.

Specific examples of the hydrophilic group in the surfactant include ahydroxy group, a hydroxyalkyl group having at least one carbon atom, ahydroxyl group, a carbonyl group, an ester group, an amino group, anamide group, an ammonium salt, thiol, sulfate, phosphate, and apolyalkyleneglycol group. Herein, the amino group may be any of aprimary amino group, a secondary amino group and a tertiary amino group.

Specific examples of the hydrophobic group in the surfactant include analkyl group having six carbon atoms, a silyl group including an alkylgroup having six carbon atoms, and a fluoroalkyl group having six carbonatoms. Herein, the alkyl group having six carbon atoms may possess anaromatic ring as a substituent. Specific examples of the alkyl groupinclude hexyl, heptyl, octyl, nonyl, decyl, undecenyl, dodecyl,tridecyl, tetradecyl, myristyl, stearyl, lauryl, palmityl, andcyclohexyl. As the aromatic ring, a phenyl group can be provided. Thissurfactant may possess at least one hydrophilic group and onehydrophobic group each in the identical molecule, or may possess twohydrophilic group and two hydrophobic group each.

Further specific examples of such the surfactant include myristyldiethanolamine, 2-hydroxyethyl-2-hydroxyldodexylamine,2-hydroxyethyl-2-hydroxytridecylamine,2-hydroxyethyl-2-hydroxytetradecylamine, pentaerythritolmonostearate,pentaerythritoldistearate, pentaerythritoltristearate,di-2-hyclroxyethyl-2-hydroxydodecylamine, alkyl (8-18 carbon atoms)benzyldimethylammonium chloride, ethylene bis alkyl (8-18 carbon atoms)amide, stearyl diethanolamide, lauryl diethanolamide, myristyldiethanolamide, and palmityl diethanolamide. Of these, amine compoundsand amide compounds having a hydroxyalkyl group are preferably used Inthe present embodiment, these compounds may be used in combination of atleast two kinds.

The adding amount of surfactant is preferably from 0.01 to 10 parts byweight to 100 parts by weight of the alicyclic hydrocarbon polymer fromthe viewpoint of efficient restriction of cloudiness of a product causedby fluctuation of temperature and humidity and the viewpoint of maintainthe high light transmittance of the product. The addition amount of thesurfactant is more preferably 0.05-5 parts by weight, with respect to100 parts by weight of the alicyclic hydrocarbon based polymer, andfurther more preferably 0.3-3 parts by weight

(Plasticizer)

Plasticizer is added as in need to adjust melt index of the copolymer.

As for plasticizer, usually known ones can be employed. For example, thefollowings are cited bis(2-ethylhexyl)adipate,bis(2-budoxyethyl)adipate, bis(2-ethylhexyl)azelate, dipropyleneglycoldibenzoate, tri-n-butyl citrate, tri-n-butylacetyl citrate, epoxidizedsoybean oil, 2-ethylhexyl epoxidized tall oil, chlorinated paraffin,tri-2-ethylhexyl phosphate, tricresyl phosphate, t-butylphenylphosphate, tri-2-ethylhexyldiphenyl phosphate, dibutyl phtalate,diisohexyl phthalate, diheptyl phthalate, dinonyl phthalate, diundecylphthalate, di-2-ethylhexyl phthalate, diisononyl phthalate, diisoclecylphthalate, ditridecyl phthalate, butylbenzyl phthalate, disyclohexylphthalate, bis(2-ethylhexyl)sebacate, (tri-2-ethylhexyl)trimelliticacid, Santicizer 278, Paraplex G40, Drapex 334F, Plastolein 9720,Mesamoll, DNODP-610, and HB-40. Selection of placticizer and its amountof addition can be determined arbitrarily so long as transmittance anddurability against change in the environment of the copolymer are notdegraded.

As the resin, cycloolefin resin is employed suitably. Specifically,LEONEX by LEON CORPORATION, APEL by Mitsui Chemicals, Inc., TOPAS madefrom TOPAS Advanced Polymers, ARTON by JSR Corporation, are cited aspreferable examples.

Further, it is preferable that a material which forms the objectivelens, has the Abbe number of 50 or more.

An objective lens of the present embodiment will be described below. Theobjective lens includes at least a central area, an intermediate areasurrounding the central area, and a peripheral area surrounding theintermediate area, on at least one optical surface. It is preferablethat the central area includes the optical axis of the objective lens.However, a small area including the optical axis may be provided as anunused area or an area for a special purpose, and the central area maybe provided to surround the small area. The central area, intermediatearea, and peripheral area are preferably formed on one optical surface.As shown in FIG. 1, it is preferable that the central area CN,intermediate area MD, peripheral area OT are provided on the sameoptical surface concentrically around the optical axis. Further, a firstoptical path difference providing structure is provided in the centralarea of the objective lens. A second optical path difference providingstructure is provided in the intermediate area. The peripheral area maybe a refractive surface, or a third optical path difference providingstructure may be provided in the peripheral area. It is preferable thateach of the central area, intermediate area, and peripheral area adjoinsto the neighboring area, however, there may be slight gaps between theadjoining areas.

The central area of the objective lens can be considered as a commonarea for the first, second and third optical discs to be used forrecording and/or reproducing information for the first optical disc, thesecond optical disc, and the third optical disc. In other words, theobjective lens converges a first light flux that passes through thecentral area so that recording and/or reproducing of information may beconducted on an information recording surface of the first optical disc,converges a second light flux that passes through the central area sothat recording and/or reproducing of information may be conducted on aninformation recording surface of the second optical disc, and convergesa third light flux that passes through the central area so thatrecording and/or reproducing of information may be conducted on aninformation recording surface of the third optical disc. Further, it ispreferable that a first optical path difference providing structurearranged in the central area corrects spherical aberration caused by adifference between thickness t1 of a protective substrate of the firstoptical disc and thickness t2 of a protective substrate of the secondoptical disc, and/or spherical aberration caused by a difference inwavelength between the first light flux and the second light flux, forthe first light flux and the second light flux both passing through thefirst optical path difference providing structure. Further, it ispreferable that the first optical path difference providing structurecorrects spherical aberration caused by a difference between thicknesst1 of a protective substrate of the first optical disc and thickness t3of a protective substrate of the third optical disc and/or sphericalaberration caused by a wavelength difference between the first lightflux and the third light flux, for the first light flux and the thirdlight flux.

An intermediate area of the objective lens can be considered as a commonarea for the first and second optical discs which are used for recordingand/or reproducing for the first optical disc and the second opticaldisc, but are not used for recording and/or reproducing for the thirdoptical disc. Namely, the objective lens converges the first light fluxthat passes through the intermediate area so that recording and/orreproducing of information may be conducted on an information recordingsurface of the first optical disc, and converges the second light fluxthat passes through the intermediate area so that recording and/orreproducing of information may be conducted on an information recordingsurface of the second optical disc. On the other hand, the objectivelens does not converge the third light flux that passes through theintermediate area so that recording and/or reproducing of informationmay be conducted on an information recording surface of the thirdoptical disc. It is preferable that the third light flux that passesthrough the intermediate area of the objective lens forms flare light onan information recording surface of the third optical disc. As shown inFIG. 2, when a spot is formed by the third light flux that has passedthrough the objective lens onto an information recording surface of thethird optical disc, the spot preferably includes a central spot portionSCN whose light density is high, an intermediate spot portion SMD whoselight density is lower than that in the central spot portion, and aperipheral spot portion SOT whose light density is higher than that inthe intermediate spot portion and is lower than that in the central spotportion. The central spot portion is used for recording and/orreproducing of information of an optical disc, while, the intermediatespot portion and the peripheral spot portion are not used for recordingand/or reproducing of information of the optical disc. In the foregoing,the peripheral spot portion is called flare light. However, also in thecase that the spot includes just the central spot portion and theperipheral spot portion but does not include the intermediate spotportion around the central spot portion, namely, in the case that a spotwhich is large in size and has weak light intensity is formed around aconverged spot, the peripheral spot portion can be called a flare light.In other words, it can be said that it is preferable that the thirdlight flux forms a peripheral spot portion on an information recordingsurface of the third optical disc.

A peripheral area of the objective lens can be considered as anexclusive area for the first optical disc that is used for recordingand/or reproducing for the first optical disc but is not used forrecording and/or reproducing for the second optical disc and the thirdoptical disc. Namely, the objective lens converges a first light fluxthat passes through the peripheral area so that recording and/orreproducing of information may be conducted on an information recordingsurface of the first optical disc. On the other hand, the objective lensdoes not converge the second light flux that passes through theperipheral area so that recording and/or reproducing of information maybe conducted on an information recording surface of the second opticaldisc, and it does not converge the third light flux that passes throughthe peripheral area so that recording and/or reproducing of informationmay be conducted on an information recording surface of the thirdoptical disc. It is preferable that the second light flux and the thirdlight flux which pass through the peripheral area of the objective lensform flare light on information recording surfaces of the second andthird optical discs. In other words, it is preferable that the secondlight flux and the third, light flux which have passed the peripheralarea of the objective lens form the peripheral spot portion.

It is preferable that first optical path difference providing structuresare provided on the area that is 70% or more of the central area of theobjective lens, and it is more preferable that the area for the firstoptical path difference providing structures is 90% or more. What ismore preferable is that the first optical path difference providingstructures are provided on the whole surface of the central area. It ispreferable that second optical path difference providing structures areprovided on the area that is 70% or more of the intermediate area of theobjective lens, and it is more preferable that the area for the secondoptical path difference providing structures is 90% or more. What ismore preferable is that the second optical path difference providingstructures are provided on the whole surface of the intermediate area.When the peripheral area has thereon the third optical path differenceproviding structure, it is preferable that third optical path differenceproviding structures are provided on the area that is 70% or more of theperipheral area of the objective lens, and it is more preferable thatthe area for the third optical path difference providing structures is90% or more. What is more preferable is that the third optical pathdifference providing structures are provided on the whole surface of theperipheral area.

Incidentally, the optical path difference providing structure mentionedin the present specification is a general term for the structure thatprovides an optical path difference to an incident light flux. Theoptical path difference providing structure also includes a phasedifference providing structure that provides a phase difference.Further, the phase difference providing structure includes a diffractivestructure. It is preferable that the optical path difference providingstructure of the present embodiment is a diffractive structure. Theoptical path difference providing structure comprises a step, and itpreferably comprises a plurality of steps. Due to the step or steps, anoptical path difference and/or a phase difference is provided to anincident light flux. An optical path difference to be provided by theoptical path difference providing structure may either be a multiple ofan integer of a wavelength of an incident light flux in terms of alength or be a multiple of a non-integer of a wavelength of an incidentlight flux. The steps may either be arranged with intervals periodicallyin the direction perpendicular to the optical axis, or be arranged withinterval non-periodically in the direction perpendicular to the opticalaxis. When the objective lens equipped with an optical path differenceproviding structure is an aspheric single lens, an incident angle of alight flux for the objective lens varies depending on a height from theoptical axis, thus, an amount of step of the optical path differenceproviding structure is slightly different from others for eachring-shaped zone. For example, when the objective lens is a convexsingle lens with an aspheric surface, it is a general trend that anamount of step of the optical path difference providing structure growsgreater as a position in the optical path difference providing structurebecomes more distant from the optical axis, even in the case of theoptical path difference providing structure that provides the constantoptical path difference.

Further, a diffractive structure mentioned in the present specificationis a general term for a structure that comprises a step or steps, forproviding a function to converge a light flux or to diverge a light fluxby a diffraction effect. For example, a diffractive structure can beformed by plural unit forms which are arranged around the optical axis,such that, when alight flux enters the respective unit forms, awavefront of the transmitted light flux is shifted at every adjoiningring-shaped zone to form a new wavefront by which light is converged ordiverged. The diffractive structure preferably includes plural steps,and the steps may either be arranged with intervals periodically in thedirection perpendicular to the optical axis, or be arranged withintervals non-periodically in the direction perpendicular to the opticalaxis. When an objective lens with a diffractive structure is provided asan aspheric single lens, an angle of a light flux entering the objectivelens varies depending on a height from the optical axis. Thereby, anamount of step of the diffractive structure slightly varies on eachring-shaped zone. For example, when the objective lens is a convexsingle lens including an aspheric surface, it is a general trend that anamount of step grows greater as a position of the step is more distantfrom the optical axis, even in a diffractive structure which generates adiffracted light fluxes with the same diffraction order.

Incidentally, it is preferable that an optical path difference providingstructure comprises a plurality of ring-shaped zones which are formed inconcentric circles whose centers are on the optical axis. Further, theoptical path difference providing structure can take generally varioussectional forms (sectional forms on the surface including an opticalaxis) which are classified roughly into a blaze-type structure and astaircase structure in terms of a sectional form including the opticalaxis.

The blaze-type structure has a form whose sectional form including theoptical axis of an optical element having an optical path differenceproviding structure are in a serrated form, as shown in FIGS. 3 a and 3b. In the example shown in FIGS. 3 a and 3 b, it is assumed that theupward of the sheet is the light source side, and the downward of thesheet is the optical disc side, and that the optical path differenceproviding structure is formed on a plane representing a base asphericsurface. In the blaze-type structure, a length of one blaze unit in thedirection perpendicular to the optical axis is called pitch P (see FIGS.3 a and 3 b). Further, a length of a step in the direction that is inparallel with the optical axis of blaze is called step amount B (seeFIG. 3 a).

The staircase structure has a form whose sectional form including anoptical axis of an optical element having an optical path differenceproviding structure has a plurality of small-staircase units (each beingcalled a stair unit), as shown in FIGS. 3 c and 3 d. Incidentally,“V-level” mentioned in the present specification means a form such thatone staircase unit of the staircase structure has ring-shaped surfaces(which is sometimes called terrace surfaces) that correspond to aperpendicular direction to the optical axis (that extend in theperpendicular direction to the optical axis), where the ring-shapedsurfaces are formed by being sectioned by the steps and are separated atevery plural ring-shaped surfaces which are V in number. Especially, astaircase structure of 3 levels or more includes small steps and largesteps.

For example, an optical path difference providing structure shown inFIG. 3 c is called a 5-level staircase structure and an optical pathdifference providing structure shown in FIG. 3 d is called a 2-levelstaircase structure (which is called also binary structure). A 2-levelstaircase structure will be explained as follows. The 2-level staircasestructure includes ring-shaped zones in concentric ringed shape aroundthe optical axis. The cross sectional form including the optical axis ofthe plural ring-shaped zones is provided with plural step surfaces Paand Pb extending parallel with the optical axis, light-source-sideterrace surfaces Pc each connecting light-source-side ends of theneighboring step surfaces Pa and Pb, and optical-disc-side terracesurfaces Pd each connecting optical-disc-side ends of the neighboringstep surfaces Pa and Pb. The light-source-side terrace surfaces Pc andthe optical-disc-side terrace surfaces Pd are arranged alternately alonga direction crossing the optical axis.

Further, in the staircase structure, a length of one staircase unit inthe direction perpendicular to the optical axis is called pitch P (seeFIGS. 3 c and 3 d). Further, a length of a step in the direction that isin parallel with the optical axis is called step amount B1 and stepamount B2. In the case of the staircase structure of 3 levels or more,large step amount B1 and small step amount B2 are in existent (see FIG.3 c).

Incidentally, it is preferable that an optical path difference providingstructure is a structure wherein a certain unit form is repeatedperiodically. The expression saying “a certain unit form is repeatedperiodically” in this case naturally includes a form wherein the sameform is repeated at the same period. Further, the expression saying “acertain unit form is repeated periodically” in this case also includes aform wherein its period becomes gradually longer or becomes graduallyshorter with regularity.

When an optical path difference providing structure has a blaze-typestructure, it has a form that a serrated forms representing unit formsare repeated. The optical path difference providing structure may have aform that the same serrated forms are repeated as shown in FIG. 3 a, orhas a shape that the pitch of a serrated form becomes gradually longeror shorter at a position that advances to be further from the opticalaxis, as shown in FIG. 3 b. In addition, The optical path differenceproviding structure may have a form that steps of the blaze-typestructure faces the direction opposite to the optical axis (center) in acertain area, and steps of the blaze-type structure faces the opticalaxis (center) in the other area, and that a transition area is providedfor switching the direction of the steps of the blaze-type structure.Incidentally, when employing a structure to switch the direction of thesteps of the blaze-type structure on the midway as stated above, itbecomes possible to enlarge a pitch of the ring-shaped zones and tocontrol a decline of transmittance that is caused by manufacturingerrors for the optical path difference providing structure.

When an optical path difference providing structure has a staircasestructure, the structure can have a form in which a 5-level staircaseunits as shown in FIG. 3 c are repeated. Further, the structure may havea form in which a pitch of a staircase units becomes gradually longer orshorter at a position that advances to be further from the optical axis.

Further, the first optical path difference providing structure and thesecond optical path difference providing structure may be formed ondifferent optical surfaces of the objective lens, respectively. However,the first optical path difference providing structure and the secondoptical path difference providing structure are preferably formed on thesame optical surface. When the third optical path difference providingstructure is further provided, it is preferable that the third opticalpath difference providing structure is formed on the same opticalsurface on which the first optical path difference providing structureand the second optical path difference providing structure are formed.By providing them on the same optical surface, it is possible to lessendecentration errors in manufacturing process, which is preferable.Further, it is preferable that the first optical path differenceproviding structure, the second optical path difference providingstructure and the third optical path difference providing structure areprovided on the surface of the objective lens facing the light-sourceside, rather than the surface of the objective lens facing theoptical-disc side. In another expression, it is preferable that thefirst optical path difference providing structure, the second opticalpath difference providing structure and the third optical pathdifference providing structure are provided on the optical surface ofthe objective lens which has a smaller absolute value of the curvatureradius.

Next, the first optical path difference providing structure provided onthe central area will be explained. The first optical path differenceproviding structure is a structure such that at least a first basicstructure and a second basic structure are overlapped with each other.

The first basic structure is of the blaze-type structure. The firstbasic structure is a blaze-type structure emits a Xth-order diffractedlight flux with a larger light amount than diffracted light fluxes withany other diffraction order, when the first light flux passes throughthe first basic structure, emits a Yth-order diffracted light flux witha larger light amount than diffracted light fluxes with any otherdiffraction order, when the second light flux passes through the firstbasic structure, and emits a Zth-order diffracted light flux with alarger light amount than diffracted light fluxes with any otherdiffraction order, when the third light flux passes through the firstbasic structure. In this case, each of X, Y and Z is an integer, and thevalue of X is an odd integer. When the value of X is an odd number whoseabsolute value is 5 or less, a step amount of the first basic structuredoes not become too great, resulting in easy manufacture, thus, it ispossible to control a loss of light amount caused by manufacturingerrors and to decrease fluctuations of diffraction efficiency in thecase of wavelength fluctuations, which is preferable.

Further, in at least apart of the first basic structure arranged aroundthe optical axis in the central area, a step or steps face the oppositedirection to the optical axis. The expression saying that “a step orsteps face the opposite direction to the optical axis” means thesituation shown in FIG. 4 b. Further, “at least apart of the first basicstructure arranged around the optical axis in the central area” means atleast a step positioned closest to the optical axis among stepsexhibiting the odd number of X. Preferably, steps exhibiting the oddnumber of X and existing in a space from the optical axis to a positionof a half of a distance from the optical axis to a boundary between thecentral area and the intermediate area, face the direction opposite tothe optical axis.

For example, apart of the first basic structure in the central area,located close to the intermediate area, may have steps facing theoptical axis. Namely, as shown in FIG. 5 b, the first basic structuremay have a form that steps positioned around the optical axis face theopposite direction to the optical axis, then, the direction of the stepsswitches on the midway, and steps positioned around the intermediatearea face the optical axis. It is preferable that all the steps of thefirst basic structure arranged in the central area face the oppositedirection to the optical axis.

By providing steps of the first basic structure in which a diffractionorder of the first light flux is an odd number and by arranging thesteps to face the direction opposite to the optical axis, it is possiblethat sufficient working distance is secured when an objective lens worksfor a CD, even in a thick objective lens having a thick axial thicknessused for achieving the compatibility of three types of optical disc ofBDs, DVDs and CDs.

Even in a thick objective lens having a thick axial thickness used forachieving the compatibility of three types of optical disc of BDs, DVDsand CDs, it is preferable that the first basic structure has a paraxialpower for the first light flux, from the viewpoint of securing awakingdistance sufficiently when the objective lens works for a CD. When anoptical path difference function of the first basic structure isexpressed by the expression (24) described later, the expression sayingthat “having a paraxial power” means that B₂h² is not 0.

The second basic structure is also a blaze-type structure. The secondbasic structure emits a Lth-order diffracted light flux with a largerlight amount than diffracted light fluxes with any other diffractionorder, when the first light flux passes through the second basicstructure, emits a Mth-order diffracted light flux with a larger lightamount than diffracted light fluxes with any other diffraction order,when the second light flux passes through the second basic structure,and emits a Nth-order diffracted light flux with a larger light amountthan diffracted light fluxes with any other diffraction order, when thethird light flux passes through the second basic structure. In thiscase, each of L, M and N is an integer, and the value of L is the eveninteger. When the value of L is an even number whose absolute value is 4or less, a step amount of the second basic structure does not become toogreat, resulting in easy manufacture, thus, it is possible to control aloss of light amount caused by manufacturing errors can be controlled,and fluctuations of diffraction efficiency caused by wavelengthfluctuations can be lowered, which is preferable.

Further, in at least apart of the second basic structure arranged aroundthe optical axis in the central area, a step or steps face the opticalaxis. The expression saying that “a step or steps face the optical axis”means the situation shown in FIG. 4 a. Further, “at least a part of thesecond basic structure arranged around the optical axis in the centralarea” means at least a step positioned closest to the optical axis amongsteps exhibiting the even number of L. Preferably, steps exhibiting theeven number of L and existing in a space from the optical axis to aposition of a half of a distance from the optical axis to a boundarybetween the central area and the intermediate area, face the directionopposite to the optical axis.

For example, apart of the second basic structure in the central area,located close to the intermediate area, may has steps facing theopposite direction to the optical axis. Namely, as shown in FIG. 5 a,the second basic structure may have a form that steps positioned aroundthe optical axis face the optical axis, then, the direction of the stepsswitches on the midway, and steps positioned around the intermediatearea face the opposite direction to the optical axis. It is preferablethat all the steps of the second basic structure arranged in the centralarea face the optical axis.

As stated above, by overlapping the first basic structure whichgenerates a diffracted light with odd diffraction order for the firstlight flux and includes steps facing the opposite direction to theoptical axis in at least a space around the optical axis in the centralarea, and the second basic structure which generates a diffracted lightwith even diffraction order for the first light flux and includes stepsfacing the optical axis in at least a space around the optical axis inthe central area, together, the following effects can be obtained. Byoverlapping these steps together, the height of steps measured after thesteps are overlapped together can be controlled not to be excessive highwhen compared with the situation that steps of the first basic structureand steps of the second basic structure are overlapped together to facethe same direction. Accordingly, it enables to restrict loss of lightamount caused due to manufacturing error and enables to restrict afluctuation of a diffraction efficiency caused when a wavelengthchanges.

It is possible to provide an objective lens which enables compatibilityof three types of optical discs of three types of BDs, DVDs and CDs, andfurther possible to provide an objective lens with well-balanced lightutilizing efficiency so as to maintain high light utilizing efficiencyfor each of the three types of optical disc of BDs, DVDs, and CDs. Forexample, it is possible to provide an objective lens wherein adiffraction efficiency for wavelength λ1 is 80% or more, a diffractionefficiency for wavelength λ2 is 60% or more and a diffraction efficiencyfor wavelength λ3 is 50% or more, from the point of view of design. Inaddition, by providing a step or steps facing the opposite direction tothe optical axis in the first basic structure, an aberration caused whena wavelength of an incident light changed to be longer, can be changedtoward, under-corrected (deficient correction) direction. Thereby, anaberration caused when a temperature of an optical pickup apparatusraises can be controlled to be small, which enables to provide anobjective lens capable of maintaining a stable performance even under atemperature change, when plastic is employed for the material of theobjective lens.

For maintaining stable properties even under the temperature change,when the objective lens is made of plastic, it is preferable that thethird order spherical aberration and fifth order spherical aberrationcaused in the objective lens when a wavelength of the incident lightflux becomes longer, are under-corrected (deficient correction).

The more preferable first optical path difference providing structure isformed by overlapping the first basic structure in which |X|, |Y| and|Z| are respectively 1, 1 and 1 and the second basic structure in which|L|, |M| and |N| are respectively 2, 1 and 1, together. Such the firstoptical path difference providing structure can have very low steps.Therefore, manufacturing errors can be reduced, and loss in light amountcan further be controlled, thus, fluctuations of diffraction efficiencycaused by wavelength fluctuations can be controlled to more excellentcondition.

From the viewpoint of a form and a step amount of the first optical pathdifference providing structure in which the first basic structure andthe second basic structure has been overlapped with each other, thefirst optical path difference providing structure wherein the firstbasic structure in which |X|, |Y| and |Z| are respectively 1, 1 and 1,and the second basic structure in which |L|, |M| and |N| arerespectively 2, 1 and 1 are overlapped together, can be expressed asfollows. It is preferable that at least apart of the first optical pathdifference providing structure arranged around the optical axis in thecentral area includes both of a step facing an opposite direction to theoptical axis and a step facing the optical axis. The step facing anopposite direction to the optical axis and the step facing the opticalaxis preferably satisfy the following expressions (1) and (2), where d11is an amount of the step facing the opposite direction to the opticalaxis, and d12 is an amount of the step facing the optical axis. Morepreferably, all steps in the central area satisfy the followingexpressions (1) and (2). When an objective lens equipped with an opticalpath difference providing structure is a convex single lens with anaspheric surface, an incident angle of a light flux entering theobjective lens varies depending on a height from the optical axis, whichresults in a trend wherein a step amount grows greater in general at aposition located further from the optical axis. The reason why the upperlimit is multiplied by 1.5 in the following expression is because anincrease of the step amount is taken into account. In the expression, nis a refractive index of the objective lens at the wavelength λ1.0.6·(λ1/(n−1))<d11<1.5·(λ1/(n−1))  (1)0.6·(λ1/(n−1))<d12<1.5·(2λ1/(n−1))  (2)

Incidentally, “at least apart of the first optical path differenceproviding structure arranged around the optical axis in the centralarea” means an optical path difference providing structure includingboth of at least a step located closest the optical axis and facing thedirection opposite to the optical axis, and a step located closest tothe optical axis and facing the direction of the optical axis.Preferably, it is an optical path difference providing structureincluding the steps existing in a space from the optical axis to aposition of a half of a distance from the optical axis to a boundarybetween the central area and the intermediate area, face the directionopposite to the optical axis.

For example, when λ1 is 390-415 nm (0.390 to 0.415 μm), and n is1.54-1.60, the above expressions can be expressed as follows.0.39 μm<d11<1.15 μm  (12)0.39 μm<d12<2.31 μm  (13)

Further, with respect to the way to overlap the first basic structureand the second basic structure together, it is preferable that the pitchof the first basic structure and the pitch of the second basic structureare adjusted such that all the steps of the second basic structure arelocated at the same positions to steps of the first basic structure, orsuch that all the steps of the first basic structure are located at thesame positions to steps of the second basic structure.

When positions of all the steps of the second basic structure areadjusted to positions of steps of the first basic structure as statedabove, it is preferable that d11 and d12 of the first optical pathdifference providing structure satisfy respectively the followingexpressions (1′) and (2′).

It is more preferable that the following expressions (1′) and (2′) aresatisfied in all over the central area.0.6·(λ1/(n−1))<d11<1.5·(λ1/(n−1))  (1′)0.6·(λ1/(n−1))<d12<1.5·(λ1/(n−1))  (2′)

Further, for example, when λ1 is 390-415 nm (0.390 to 0.415 μm) and n is1.54-1.60, the expressions above can be expressed as follows.0.39 μm<d11<1.15 μm  (12)0.39 μm<d12<1.15 μm  (13)

It is more preferable that the following expressions are satisfied. Itis further more preferable that the following expressions (1″) and (2″)are satisfied in all over the central area0.9·(λ1/(n−1))<d11<1.5·(λ1/(n−1))  (1″)0.9·(λ1/(n−1))<d12<1.5·(λ1/(n−1))  (2″)

Further, for example, λ1 is 390-415 nm (0.390 to 0.415 μm) and n is1.54-1.60, the expressions above can be expressed as follows.0.59 μm<d11<1.15 μm  (12″)0.59 μm<d12<1.15 μm  (13″)

By providing a first optical path difference providing structure formedby overlapping the first basic structure wherein |X|, |Y| and |Z| arerespectively 1, 1 and 1 and the second basic structure wherein |L|, |M|and |N| are respectively 2, 1 and 1, together, the first basic structurecan exhibit under-corrected (deficient correction) aberration when thewavelength of the incident light flux becomes longer, and the secondbasic structure can exhibit over-corrected (excessive correction). Suchthe structure do not exhibit wavelength characteristic which isexcessively under-corrected or is excessively over-corrected, butexhibit wavelength characteristic which is under-corrected to a properdegree. As for the “wavelength characteristic which is under-correctedto a proper degree”, it is preferable that an absolute value of λrms is150 or less. Due to this, it is possible to control aberration changecaused by temperature change to be small even in the case when anobjective lens is made of plastic, which is preferable from a point ofview of the foregoing.

From the viewpoint of obtaining “wavelength characteristic which isunder-corrected to a proper degree” as stated above, it is preferablethat contribution of the first basic structure is dominant compared withthe second basic structure. From a point of view to make contribution ofthe first basic structure to be dominant over the second basicstructure, it is preferable that an average pitch of the first basicstructure is smaller than that of the second basic structure. In otherexpressions, it is possible to express that a pitch of steps facing thedirection opposite to the optical axis is smaller than a pitch of stepsfacing the optical axis, or it is possible to express that the number ofsteps facing the direction opposite to the optical axis is more than thenumber of steps facing the optical axis, in the first optical pathdifference providing structure. Incidentally, it is preferable that anaverage pitch of the first basic structure is one-fourth or less of anaverage pitch of the second basic structure. It is more preferable thatan average pitch of the first basic structure is one-sixth or less of anaverage pitch of the second basic structure. By making an average pitchof the first basic structure to be one-fourth (or one-sixth, preferably)of an average pitch of the second basic structure, it is possible tomake wavelength characteristics to be “under-corrected to a properdegree”, and it is also preferable from a point of view to securesufficient working distance in the case of a CD. In other expressions,it is possible to express that it is preferable that the number of stepsfacing the direction opposite to the optical axis is four times or morethe number of steps facing the optical axis, in the first optical pathdifference providing structure in the central area. Six times or more ismore preferable.

Further, it is preferable that a minimum pitch of the first optical pathdifference providing structure is 15 μm or less. From the viewpoint, theratio p/fl, which is a ratio of the minimum pitch “p” of the firstoptical path difference providing structure to the focal length “fl” ofthe objective lens for the first wavelength is preferably 0.004 or less.The minimum pitch of 10 μm or less is more preferable. It is preferablethat an average pitch of the first optical path difference providingstructure is 30 μm or less. It is more preferable that the average pitchis 20 μm or less. By providing such the structure, it is possible toobtain wavelength characteristics to be under-corrected to a properdegree as stated above, and it is possible to keep a best focus positionfor necessary light used for recording and/or reproducing informationfor the third optical disc and a best focus position for unwanted lightthat is not used for recording and/or reproducing information for thethird optical disc to be away from each other, and detection error canbe reduced. Meanwhile, an average pitch is a value obtained by addingall pitches of the first optical path difference providing structure inthe central area and by dividing it by the number of steps of the firstoptical path difference providing structure in the central area.

The objective lens of the present embodiment preferably exhibits alongitudinal chromatic aberration of 0.9 μm/nm or less, and morepreferably exhibits that of 0.8 μm/nm or less. When the pitch of thefirst basic structure is excessively small, the longitudinal chromaticaberration deteriorates. Therefore, the objective lens is preferablydesigned in view of the pitch so as not to provide the longitudinalchromatic aberration of more than 0.9 μm/nm. From the viewpoint, theratio p/fl, which is a ratio of the minimum pitch “p” of the firstoptical path difference providing structure to the focal length “fl” ofthe objective lens for the first wavelength λ1, is preferably 0.002 ormore. On the other hand, the longitudinal chromatic aberration ispreferably 0.4 μm/nm or more, in order to secure a sufficient workingdistance for CDs.

It is preferable that a first best focus position where light intensityof a spot formed by the third light flux is strongest and a second bestfocus position where light intensity of a spot formed by the third lightflux is second strongest satisfy the following conditional expression(14) with the third light flux that has passed through the first opticalpath difference providing structure. Meanwhile, the best focus positionmentioned in this case is one that indicates a position where a beamwaist becomes a minimum in a certain defocusing range. The first bestfocus position is a best focus position for necessary light used forrecording and/or reproducing for the third optical disc, and the secondbest focus position is a best focus position for a light flux having thelargest amount of light amount among unwanted light fluxes which are notused for recording and/or reproducing for the third optical disc.0.05≦L/fl3≦0.35  (14)

In the expression above, fl3 (mm) is a focal length of the third lightflux that passes through the first optical path difference providingstructure and forms a first best focus, and L (mm) indicates a focallength between the first best focus and the second best focus.

It is more preferable that the following conditional expression (14′) issatisfied0.10≦L/fl3≦0.25  (14)

A preferable example of the first optical path difference providingstructure explained above is shown in FIG. 6. However FIG. 6 shows astructure that the first optical path difference structure ODS1 isprovided on a flat plate for convenience, the first optical pathdifference structure may also be provided on a convex single lens withan aspheric surface. The first basic structure BS1 wherein |X|, |Y| and|Z| are respectively 1, 1 and 1 is overlapped with the second basicstructure BS2 wherein |L|, |M| and |N| are respectively 2, 1 and 1.Steps of the second basic structure BS2 face the direction of opticalaxis OA, and steps of the first basic structure BS1 face the directionopposite to the optical axis. As can be seen from FIG. 6, the pitch ofthe first basic structure BS1 and the pitch of the second basicstructure BS2 are adjusted such that all the steps of the second basicstructure are located at the same positions to steps of the first basicstructure. In the present example, d11=λl/(n−1) holds and d12=λ1/(n−1)holds. In the present example, when λ1=405 nm (0.405 μm) and n=15592hold, d11=d12=0.72 μm holds. In the present example, an average pitch ofthe first basic structure is smaller than that of the second basicstructure, and the number of steps facing the direction opposite to theoptical axis of the first basic structure is more than that of stepsfacing the optical axis of the second basic structure.

Next, the second optical path difference providing structure provided inthe intermediate area will be explained. It is preferable that thesecond optical path difference providing structure is a structure inwhich at least two basic structures including the third basic structureand the fourth basic structures are overlapped together.

The third basic structure emits an Ath-order diffracted light flux witha larger light amount than diffracted light fluxes with any otherdiffraction order, when the first light flux passes through the thirdbasic structure, emits a Bth-order diffracted light flux with a largerlight amount than diffracted light fluxes with any other diffractionorder, when the second light flux passes through the third basicstructure, and emits a Cth-order diffracted light flux with a largerlight amount than diffracted light fluxes with any other diffractionorder, when the third light flux passes through the third basicstructure. The fourth basic structure emits a Dth-order diffracted lightflux with a larger light amount than diffracted light fluxes with anyother diffraction order, when the first light flux passes through thefourth basic structure, emits a Eth-order diffracted light flux with alarger light amount than diffracted light fluxes with any otherdiffraction order, when the second light flux passes through the fourthbasic structure, and emits a Fth-order diffracted light flux with alarger light amount than diffracted light fluxes with any otherdiffraction order, when the third light flux passes through the fourthbasic structure. In this case, each of A, B, C, D, E and F is aninteger.

The first optical path difference providing structure and the secondoptical path difference providing structure preferably satisfy thefollowing expressions (15), (16), (17) and (18). Owing to this, a phasedifference generated in the optical path difference providing structuresin the central area and a phase difference generated in the optical pathdifference providing structure in the intermediate area can be made tobe almost equal, and a phase shift between the central area and theintermediate area can be reduced accordingly, which is preferable.X=A  (15)Y=B  (16)L=D  (17)M=E  (18)

It is more preferable that Z=C and N=F are also satisfied. In otherwords, it is preferable that the first basic structure is the same asthe third basic structure in terms of a structure and the second basicstructure is the same as the fourth basic structure in terms of astructure.

When the third basic structure and the fourth basic structure are thesame as the first basic structure and the second basic structure,respectively, it is preferable that a fifth basic structure ispreferably overlapped with the third and fourth basic structures, in thesecond optical path difference providing structure.

At that time, it is preferable that the fifth basic structure emits a0th-order diffracted light flux with a larger light amount thandiffracted light fluxes with any other diffraction order, when the firstlight flux passes through the fifth basic structure, emits a 0th-orderdiffracted light flux with a larger light amount than diffracted lightfluxes with any other diffraction order, when the second light fluxpasses through the fifth basic structure, and emits a Gth-orderdiffracted light flux with a larger light amount than diffracted lightfluxes with any other diffraction order, when the third light fluxpasses through the fifth basic structure. In this case, G is an integerexcluding 0. By overlapping such the fifth basic structure with theother basic structure, it is possible to form flare light on aninformation recording surface of the third optical disc with respect toonly the third light flux, without having an influence on the firstlight flux and the second light flux each passing the intermediate areaand generating a phase shift at the boundary of the central area and theintermediate area. Thereby, an influence of the unwanted light on theconverged spot can be reduced.

It is preferable that G is ±1. When G is ±1, it is preferable that thefifth basic structure is a two-revel staircase structure (which is alsocalled a binary structure) as shown in FIG. 3 d.

When the fifth basic structure is a two-level staircase structure, it ispreferable that step amount LB1 measured along its optical axis is thestep amount that provides an optical path difference equivalent to 5times the first wavelength λ1 for the first light flux, or the stepamount that provides an optical path difference equivalent to 3 timesthe first wavelength λ1 for the first light flux. When the two-levelstaircase structure provides an optical path difference equivalent to 5times the first wavelength λ1 for the first light flux, an influence ofan unwanted light caused when information is recorded and/or reproducedfor CD can be reduced substantially, which is preferable. On the otherhand, by forming the two-level staircase structure into a structure toprovide an optical path difference equivalent to 3 times the firstwavelength λ1 for the first light flux, it is possible to lower a heightof the fifth basic structure, thus, it is easy to manufacture, andmanufacturing loss can be reduced, which is preferable from a viewpointto prevent a decline of alight utilizing efficiency. It is alsopreferable from the viewpoint of controlling fluctuations of diffractionefficiency under the wavelength change to be small.

Namely, it is preferable that the step amount LB1 of the fifth basicstructure satisfies the following conditional expressions (19) and (20).0.9·(5·λ1/(n−1))<LB1<1.5·(5·λ1/(n−1))  (19)0.9·(3·λ1/(n−1))<LB1<1.5·(3·λ1/(n−1))  (20)

Further, when λ1 is 390-415 nm (0.390-0.415 μm) and n is 1.54-1.60, theexpressions above can be expressed as follows.2.92 μm<LB1<5.77 μm  (21)1.75 μm<LB1<3.46 μm  (22)

Therefore, a preferable second optical path difference providingstructure becomes a structure wherein a binary structure satisfying G=±1is overlapped with a structure that is equivalent to the aforesaidpreferable first optical path difference providing structure.

Further, the second optical path difference providing structurepreferably comprises steps on upper terrace surface Pc of a two-levelstaircase structure shown in intermediate area MD in FIG. 10. What isfurther preferable is that a plurality of steps are provided. It ispreferable that these steps are derived from plural third basicstructures and a single fourth basic structure.

By providing plural steps of the third basic structure on the upperterrace surface of the two-level staircase structure, it becomes easyfor resins to run up to an end portion of a mold of the two-levelstaircase structure. Thereby, transfer characteristics are improved, aloss in manufacturing can be decreased and a decline of lightutilization efficiency can be prevented. In addition, it becomespossible to locate a converging position of unwanted diffracted light tobe farther from a converging position of necessary diffracted light.Thereby, a detection error caused by unwanted diffracted light convergedon a light-receiving element, can be avoided, which is preferable.

It is preferable that the smallest pitch of the fifth basic structure is10 μm or more. The pitch is preferably 100 μm or less. By employing thetwo-level staircase structure whose step amount tends to be higher, inthe intermediate area rather than the central area, it becomes possibleto broaden a pitch, so that resin may easily reaches the deep portion ofa mold in injection molding process, and thereby, to reduce amanufacturing loss.

Next, when providing the third optical path difference providingstructure in the peripheral area, an arbitral optical path differenceproviding structure can be provided. The third optical path differenceproviding structure preferably comprises a sixth basic structure. Thesixth basic structure emits an Pth-order diffracted light flux with alarger light amount than diffracted light fluxes with any otherdiffraction order, when the sixth light flux passes through the thirdbasic structure, emits a Qth-order diffracted light flux with a largerlight amount than diffracted light fluxes with any other diffractionorder, when the second light flux passes through the sixth basicstructure, and emits a Rth-order diffracted light flux with a largerlight amount than diffracted light fluxes with any other diffractionorder, when the third light flux passes through the sixth basicstructure. In order to control a fluctuation of diffraction efficiencycaused under wavelength change, the value of P is preferably 5 or less,and more preferably 2 or less.

A numerical aperture of the objective lens on the image side that isneeded for reproducing and/or recording of information for the firstoptical disc is represented by NA1, a numerical aperture of theobjective lens on the image side that is needed for reproducing and/orrecording of information for the second optical disc is represented byNA2 (NA1>NA2) and a numerical aperture of the objective lens on theimage side that is needed for reproducing and/or recording ofinformation for the third optical disc is represented by NA3 (NA2>NA3).NA1 is preferably 0.75 or more and is 0.9 or less, and it is 0.8 or moreand is 0.9 or less more preferably. It is especially preferable that NA1is 0.85. NA2 is preferably 0.55 or more and is 0.7 or less. It isespecially preferable that NA2 is 0.60 or 0.65. Further, NA3 ispreferably 0.4 or more and is 055 or less. It is especially preferablethat NM is 0.45 or 053.

It is preferable that a boundary between a central area and anintermediate area of the objective lens is formed on a portion thatcorresponds to a range from 0.9·NM3 or more to 12·NM or less (morepreferably, 0.95·NA3 or more to 1.15·NA3 or less) in the case of usingthe third light flux. More preferably, a boundary between a central areaand an intermediate area is formed on a portion corresponding to NA3. Itis further preferable that a boundary between an intermediate area and aperipheral area is formed on a portion corresponding to a range from0.9·NA2 or more to 1.2·NA2 or less (more preferably, 0.95·NA2 or more to1.15·NA2 or less) in the case of using the second light flux. Morepreferably, a boundary between an intermediate area and a peripheralarea of the objective lens is formed on a portion corresponding to NA2.

When the third light flux that has passed through the objective lens isconverges on an information recording surface of the third optical disc,it is preferable that spherical aberration has at least onediscontinuous portion. In that case, it is preferable that thediscontinuous portion is in existent in a range from 0.9·NA3 or more to1.2·NA3 or less (more preferably, 0.95·NA3 or more to 1.15·NA3 or less)in the case of using the third light flux.

The objective lens preferably satisfies the following conditionalexpression (3).1.0≦d/f≦1.5  (3)

In the expression above, d represents a thickness (mm) of the objectivelens on the optical axis, and f represents a focal length of theobjective lens in the first light flux.

When coping with an optical disc used with a short wavelength and highNA like a BD, there are caused problems that astigmatism tends to becaused and decentration coma tends to be caused. However, when theexpression (3) is satisfied, it is possible to control occurrence ofastigmatism and decentration coma.

Further, when the expression (3) is satisfied, an objective lens becomesa thick objective lens whose thickness on the axis is great, thus,working distance in recording and/or reproducing for CDs tends to beshort. However, the present embodiment employs the first optical pathdifference providing structure to secure a sufficient working distancein recording and/or reproducing for CD can be secured sufficiently.Therefore, its effect becomes remarkable.

Each of the first light flux, the second light flux and the third lightflux may enter into the objective lens as a parallel light flux, or mayenter into the objective lens as a divergent light flux or a convergentlight flux. It is preferable that all of the first light flux, thesecond light flux and the third light flux enter into the objective lensas parallel light fluxes in order to prevent generation of coma even inthe tracking operation. By employing the first optical path differenceproviding structure of the present embodiment, all of the first lightflux, the second light flux and the third light flux can enter into theobjective lens as parallel light fluxes or almost parallel light fluxes.Thereby, its effect becomes remarkable. When the first light flux is aparallel light flux or an almost parallel light flux, it is preferablethat the magnification m1 of the objective lens when the first lightflux enters into the objective lens satisfies the following expression(4).−0.01<m1<0.01  (4)

When the second light flux enters the objective lens as a parallel lightflux or an almost parallel light flux, it is preferable that themagnification m2 of the objective lens when the second light flux entersinto the objective lens satisfies the following expression (5).−0.01<m2<0.01  (5)

When the second light flux enters the objective lens as a divergentlight flux, it is preferable that the magnification m1 of the objectivelens when the second light flux enters into the objective lens satisfiesthe following expression (5′).−0.025<m2≦−0.01  (5′)

When the third light flux enters the objective lens as a parallel lightflux or an almost parallel light flux, it is preferable that themagnification m3 of the objective lens when the third light flux entersinto the objective lens satisfies the following expression (6).−0.01<m3<0.01  (6)

On the one hand, when third light flux enters the objective lens as adivergent light flux, it is preferable that the magnification m3 of theobjective lens when the third light flux enters into the objective lenssatisfies the following expression (6′).−0.025<m3≦−0.01  (6′)

Further, it is preferable that the working distance (WD) of theobjective lens when the third optical disc is used, is 0.15 mm or mole,and is 1.5 mm or less. The working distance of the objective lens whenthe third optical disc is preferably 0.3 mm or more, and 0.9 mm or less.Next, it is preferable that the working distance (WD) of the objectivelens when the second optical disc is used, is 0.2 mm or more, and is 1.3mm or less. The working distance is preferably that the working distance(WD) of the objective lens when the first optical disc is used, is 0.25mm or more, and is 1.0 mm or less.

An optical information recording and reproducing apparatus comprises anoptical disc drive apparatus including the above optical pickupapparatus.

Herein, the optical disc drive apparatus installed in the opticalinformation recording and reproducing apparatus will be described. Thereis provided the optical disc drive apparatus employing a system suchthat there is a tray which can hold an optical disc with the opticaldisc placed thereon and only the tray is taken out from the main body ofthe optical information recording and reproducing apparatus which housesan optical pickup apparatus therein; and a system such that the mainbody of the optical disc drive apparatus which houses an optical pickupapparatus therein is taken out.

The optical information recording and reproducing apparatus using eachof the above described systems, is generally provided with the followingcomponent members: an optical pickup apparatus housed in a housing; adrive source of the optical pickup apparatus such as a seek-motor bywhich the optical pickup apparatus is moved together with the housingtoward the inner periphery or outer periphery of the optical disc;traveling means for the optical pickup apparatus, including a guide milfor guiding the housing of the optical pickup apparatus toward the innerperiphery or outer periphery of the optical disc; and a spindle motorfor rotation drive of the optical disc. However, the component membersof the optical information recording and reproducing apparatus are notlimited to those.

The optical information recording and reproducing apparatus employingthe former system is preferably provided with, other than thosecomponent members, a tray which can hold an optical disc under thecondition that the optical disc is placed thereon, and a loadingmechanism for slidably moving the tray. It is preferable that theoptical information recording and reproducing apparatus employing thelatter system does not include the tray and loading mechanism, andrespective component members are provided in a drawer corresponding tochassis which can be taken out outside.

According to the above embodiments, even in a thick objective lens whichis used for achieving compatibility of three types of optical disc ofBDs, DVDs, and CDs and is thick along the optical axis, a sufficientworking distance can be secured when the objective lens works for a CD.Further, the height of steps of the optical path difference providingstructure can be controlled not to be excessive high, which enables torestrict loss of light amount cased due to manufacturing error, andenables to restrict a fluctuation of a diffraction efficiency causedwhen a wavelength changes. Further, the above embodiments can provide anobjective lens with well-balanced light utilizing efficiencies so as tomaintain high light utilizing efficiency for each of the three types ofoptical disc of BDs, DVDs, and CDs. The embodiments are advantageous inthe way of downsizing. Further, an aberration caused when a temperatureof an optical pickup apparatus raises can be controlled to be small,which enables to provide an objective lens which can maintain a stableperformance even under a temperature change. These effects enable torecord and/or reproduce information compatibly for the three types ofoptical discs of BDs, DVDs, and CDs with a common objective lens.

Preferred embodiments of the present invention will be described below,with referring the drawings. FIG. 7 is a diagram schematically showingoptical pickup apparatus PU1 of the present embodiment capable ofrecording and/or reproducing information adequately for BDs, DVDs andCDs which are different optical discs. The optical pickup apparatus PU1can be mounted in the optical information recording and reproducingapparatus. Herein, the first optical disc is a BD, the second opticaldisc is a DVD, and the third optical disc is a CD. Hereupon, the presentinvention is not limited to the present embodiment.

Optical pickup apparatus PU1 comprises objective lens OL, quarterwavelength plate QWP, collimation lens COL, polarization beam splitterBS, dichroic prism DP, first semiconductor laser LD1 (first lightsource), laser unit LDP, sensor lens SEN, and light-receiving element PDas a light-receiving element. The first semiconductor laser LD1 (firstlight source) emits a laser light flux with a wavelength of 405 nm (thefirst light flux) when information is recordedkproduced for BDs. Thelaser unit LDP includes second semiconductor laser LD2 (second lightsource) emitting a laser light flux with a wavelength of 660 nm (secondlight flux) when information is recorded/reproduced for DVDs, and thirdsemiconductor laser LD3 (third light source) emitting a laser light fluxwith a wavelength of 785 nm (third light flux) when information isrecorded/reproduced for CDs, which are unitized in one body.

As shown in FIG. 1, objective lens OL which relates to the presentembodiment is provided as a single lens includes an aspheric opticalsurface at the light source side on which central area CN including theoptical axis, intermediate area MD arranged around the central area; andperipheral area OT arranged around the peripheral area are formed intoconcentric rings around the optical axis. The above-described firstoptical path difference providing structure is formed in central areaCN, and the above-described second optical path difference providingstructure is formed in intermediate area MD. In peripheral area OT, thethird optical path difference providing structure is formed. In thepresent embodiment, the third optical path difference providingstructure is a blaze-type diffractive structure. The objective lens ofthe present embodiment is a plastic lens. The first optical pathdifference providing structure formed in central area CD is a structurein which the first basic structure and the second basic structure areoverlapped together as shown in FIG. 6. The first basic structure emitsa −1st order diffracted light flux with a larger light amount thandiffracted light fluxes with any other diffraction order, when the firstlight flux passes through the first basic structure, emits a −1st orderdiffracted light flux with a larger light amount than diffracted lightfluxes with any other diffraction order, when the second light fluxpasses through the first basic structure, and emits −1st orderdiffracted light flux with a larger light amount than diffracted lightfluxes with any other diffraction order, when the third light fluxpasses through the first basic structure. At least a part of the firstbasic structure arranged around an optical axis in the central area CNincludes a step or steps facing an opposite direction to the opticalaxis (namely, having a negative power). The second basic structure emitsa second-order diffracted light flux with a larger light amount thandiffracted light fluxes with any other diffraction order, when the firstlight flux passes through the second basic structure, which emits afirst-order diffracted light flux with a larger light amount thandiffracted light fluxes with any other diffraction order, when thesecond light flux passes through the second basic structure, and whichemits a first-order diffracted light flux with a larger light amountthan diffracted light fluxes with any other diffraction order, when thethird light flux passes through the second basic structure. At least apart of the second basic structure arranged around the optical axis inthe central area CN includes a step or steps facing the optical axis(Namely, having a positive power). The objective lens exhibits alongitudinal chromatic aberration of 0.9 μm/nm or less.

Blue-violet semiconductor laser LD1 emits a first light flux (λ1=405 nm)which is a divergent light flux. As illustrated by solid lines, thedivergent light flux passes through dichroic prism DP and polarizationbeam splitter BS, and is converted into a collimated light flux bycollimation lens COL. Quarter wavelength plate QWP converts thepolarization of the collimated light from linear polarization tocircular polarization. Then, the diameter of the resulting light flux isregulated by a stop and the light flux enters objective lens OL. A lightflux converged by the central area, intermediate area, and peripheralarea of objective lens OL, forms a spot on information recording surfaceRL1 of a BD through protective substrate PL1 with thickness of 0.1 mm.

The light flux on information recording surface RL1 is reflected andmodulated by the information pit on the information recording surfaceRL1. The reflected light flux passes through objective lens OL and thestop again, and quarter wavelength plate QWP converts the polarizationof the light flux from circular polarization to linear polarization.Then, collimation lens COL converts the light flux into a convergentlight flux. The convergent light flux is reflected by polarization beamsplitter BS and is converged on a light-receiving surface of lightreceiving element PD through sensor lens SEN. Then, information recordedin a BD can be read based on the output signal of light-receivingelement PD, by performing focusing and tracking operations for objectivelens OL using biaxial actuator AC1. When the wavelength changes in thefirst light flux or when information is recorded and/or reproduced forBD including plural information recording layers, collimation lens COLas a magnification changing means is displaced in the direction of theoptical axis to change a divergent angle or convergent angle of a lightflux entering objective lens OL. Thereby, spherical aberration causedbecause of the wavelength change or the difference of the informationrecording layers can be corrected.

Semiconductor laser LD2 in laser unit LDP emits a second light flux(λ2=658 nm) which is a divergent light flux. As illustrated by dottedlines, the emitted divergent light flux is reflected by dichroic prismDP and passes polarization beam splitter BS and collimation lens COL.Then, quarter wavelength plate QWP converts the polarization of thelight flux from linear polarization to circular polarization, and theresulting light flux enters objective lens OL. Herein, a light fluxconverged by the central area and the intermediate area of objectivelens OL becomes a spot formed on information recording surface PL2 of aDVD through protective substrate PL2 with thickness of 0.6 mm, to form acentral spot portion, where a light flux passing through theintermediate area and the peripheral area is formed into flare light toform a peripheral spot portion.

The light flux on information recording surface RL2 is reflected andmodulated by the information pit on the information recording surfaceRL2. The reflection light flux passes through objective lens OL again,and quarter wavelength plate QWP converts the polarization of the lightflux from circular polarization to linear polarization. The resultinglight flux is formed into a convergent light flux by collimation lensCOL, and reflected by polarization beam splitter BS. Then, the lightflux is converged on a light-receiving surface of light-receivingelement PD through sensor lens SEN. Then, the information recorded in aDVD can be read by using the output signal of light-receiving elementPD.

Semiconductor laser LD3 in laser unit LDP emits a third light flux(λ3=785 nm) which is a divergent light flux. As illustrated by longdashed short dashed line, the divergent light flux is reflected bydichroic prism DP, and passes polarization beam splitter BS andcollimation lens COL. Then, quarter wavelength plate QWP converts thepolarization of the light flux from linear polarization to circularpolarization and the resulting light flux enters objective lens OL.Herein, the incident light flux is converged by the central area ofobjective lens OL forms a spot on information recording surface PL3 ofCD through protective substrate PL2 with thickness of 1.2 mm, where alight flux passing through the intermediate area and the peripheral areais formed into a flare light to form a peripheral spot portion.

The light flux on information recording surface RL3 is reflected andmodulated by the information pit on the information recording surfaceRL3. The reflection light flux passes through objective lens OL again,and quarter wavelength plate QWP converts the polarization of the lightflux from circular polarization from linear polarization. The resultinglight is formed into a convergent light flux by collimation lens COL andreflected by polarization beam splitter BS. Then, the light flux isconverged on a light-receiving surface of the light-receiving element PDthrough sensor lens SN. Then, information recorded in a CD can be readby using output signal of the third light-receiving element PD.

EXAMPLES

Next, an example which can be used for the above described embodimentwill be described. Hereinafter (including lens data in a table), thepower of 10 will be expressed as by using “E” (For example, 25×10⁻³ willbe expressed as 2.5E-3). Each optical surface of the objective lens isformed as an aspheric surface, which has a symmetric shape around theoptical axis defined by a mathematical expression obtained by assigningcorresponding values of coefficients to the following expression (23).

$\begin{matrix}{{X(h)} = {\frac{\left( {h^{2}/r} \right)}{1 + \sqrt{1 - {\left( {1 + \kappa} \right)\left( {h/r} \right)^{2}}}} + {\sum\limits_{i = 0}^{10}{A_{2\; i}h^{2\; i}}}}} & (23)\end{matrix}$

Herein, X(h) is an axis along the optical axis (the direction oftraveling light is defined as a positive direction), κ is a conicconstant, A_(2i) is an aspheric coefficient, his a height from theoptical axis, and r is a paraxial curvature radius.

In a example employing a diffractive structure, an optical pathdifference provided by the diffractive structure to each of the lightfluxes with respective wavelengths is defined by a mathematicalexpression obtained by assigning corresponding coefficients to thefollowing optical path difference function.Φ=mλΣB _(2i) h ^(2i)  (24)

-   -   (Unit mm)

In the expression, his a height from the optical axis, λ is a wavelengthof an incident light flux, m is a diffraction order, and B₂ is acoefficient of the optical path difference function.

Example 1

An objective lens of Example 1 is a plastic single lens. A schematicdiagram of the first optical path difference providing structure inExample 1 is shown in FIG. 6, where the diagram in FIG. 6 is just aschematic diagram and it is different from the actual form of Example 1.The first optical path difference providing structure of Example 1 is anoptical path difference providing structure wherein first basicstructure BS1 representing ablaze-type diffractive structure in which|X|, |Y| and |Z| are respectively 1, 1 and 1 is overlapped with secondbasic structure BS2 representing a blaze-type diffractive structure inwhich |L|, |M| and |N| are respectively 2, 1 and 1. Steps in the secondbasic structure BS2 face the direction of the optical axis OA, and stepsin the first basic structure BS1 face the direction opposite to theoptical axis OA. In addition, a pitch of first basic structure BS1 isadjusted to the second basic structure BS2, so that positions of all thesteps in the second basic structure agree with positions of steps of thefirst basic structure. Further, an average pitch of first basicstructure BS1 is smaller than an average pitch of the second basicstructure BS2, and the number of steps facing the direction opposite tothe optical axis of the first basic structure is more than that of stepsfacing the direction of the optical axis in the second basic structure.

The first optical path difference providing structure in Example 1satisfies the following expressions (1″) and (2″). The symbol d11represents an amount of steps facing the direction opposite to theoptical axis, and d12 represents an amount of steps facing the directionof the optical axis.0.9·(λ1/(n−1))<d11<1.5·(λ1/(n−1))  (1″)0.9·(λ1/(n−1))<d12<1.5·(λ1/(n−1))  (2″)

Since λ1 is 405 nm (0.405 μm) and n is 1.5592 in Example 1, step amountd11 and step amount d12 satisfy the following expressions.0.65 μm<d11<1.09 μm0.65 μm<d12<1.09 μm

The second optical path difference providing structure in Example 1 isan optical path difference providing structure wherein the fifth basicstructure is overlapped with a structure in which the third basicstructure equal to the first basic structure and the fourth basicstructure equal to the second basic structure are overlapped together.The fifth basic structure in Example 1 is a two-level staircasediffractive structure (binary structure) which emits a 0th-orderdiffracted light flux with a larger light amount than diffracted lightfluxes with any other diffraction order, when the first light fluxpasses through the fifth basic structure, emits a 0th-order diffractedlight flux with a larger light amount than diffracted light fluxes withany other diffraction order, when the second light flux passes throughthe fifth basic structure, and emits ±first order diffracted light fluxwith a larger light amount than diffracted light fluxes with any otherdiffraction order, when the third light flux passes through the fifthbasic structure.

Table 1 shows lens data of Example 1.

TABLE 1 Focal length of objective lens f₁ = 2.20 mm f₂ = 2.38 mm f₃ =2.45 mm Numerical Aperture NA1: 0.85 NA2: 0.60 NA3: 0.47 Magnificationm1: 0 m2: 0 m3: 0 i-th surface ri di (405 nm) ni (405 nm) di (660 nm) ni(660 nm) di (785 nm) ni (785 nm) 0 ∞ ∞ ∞ 1 (Stop 0.0 0.0 0.0 diameter)(φ3.74 mm) (φ2.87 mm) (φ2.30 mm) 2-1 1.3099 2.670 1.5414 2.670 1.52252.670 1.5193 2-2 1.5097 2-3 1.4723 3 −2.3669 0.721 0.646 0.359 4 ∞0.0875 1.6196 0.600 1.5773 1.200 1.5709 5 ∞ Surface no. 2-1 2-2 2-3 3Area h ≦ 1.180 1.180 ≦ h ≦ 1.45 1.45 ≦ h ≦ 1.87 Aspheric κ −8.7226E−01−3.7413E−01 −5.9930E−01 −3.3091E+01 surface A0 0.0000E+00 2.4521E−022.2268E−02 0.0000E+00 coefficient A4 7.9383E−03 2.3786E−02 1.9406E−021.0060E−01 A6 5.4165E−03 −1.8940E−03 −1.0124E−04 −9.9722E−02 A83.1408E−04 −3.0942E−04 2.4046E−03 7.7657E−02 A10 −1.3516E−03 −2.2047E−03−1.5974E−03 −4.3120E−02 A12 5.1208E−04 5.9886E−04 2.3273E−04 1.4491E−02A14 7.0800E−04 3.2379E−04 2.3920E−04 −2.5798E−03 A16 −7.9609E−04−2.2751E−04 −1.6547E−04 1.8060E−04 A18 3.2163E−04 8.4526E−05 4.5018E−050.0000E+00 A20 −4.3814E−05 −1.5640E−05 −4.6358E−06 0.0000E+00Diffraction order 1/1/1 1/1/1 2/1/1 First B2 6.3821E+01 6.4208E+011.4185E+01 optical B4 −6.0360E+00 −6.3479E+00 1.7360E+00 path B63.1232E+00 2.4210E+00 −1.7979E−01 difference B8 −1.3062E+00 −5.1565E−01−1.0132E−01 function B10 2.5156E−01 5.8699E−02 −4.6009E−02 Diffractionorder 2/1/1 2/1/1 — Second B2 −7.6263E+00 −7.6722E+00 — optical B4−3.7264E+00 −4.1548E+00 — path B6 1.5761E+00 1.4111E+00 — difference B8−9.7167E−01 −3.8603E−01 — function B10 2.3713E−01 7.2573E−02 —Diffraction order — 0/0/±1 — Third B2 — −9.4827E+01 — optical B4 —1.9720E+02 — path B6 — −1.5525E+02 — difference B8 — 5.5362E+01 —function B10 — −7.4420E+00 — Wavelength Characteristic +5 nm ΔSA3:−0.105 ΔSA5: −0.024 SAH: 0.015 |d(n − 1)/λ₁*N|: 45

Further, Tables 2 to 6 show data of an actual form of the objective lenswhich is designed based on lens data of Example 1. The data of actualform of each ring-shaped zone is obtained by substituting data shown inTables 2 to 6 in the numerical expression (25). In Tables 2 to 6, hsrepresents a height at which each ring-shaped zone starts, and hlrepresents a height at which each ring shaped zone ends.x=A ₀ +A ₂ ×h ² +A ₄ ×h ⁴ +A ₆ ×h ⁶  (25)

The symbol h represents a height from the optical axis in the directionperpendicular to the optical axis. Further, FIG. 10 is a sectional viewschematically showing the first optical path difference providingstructure, the second optical path difference providing structure, andthe third optical path difference providing structure of Example 1 whichare formed on a flatplate. The symbol CN represents a central area onwhich the first optical path difference providing structure is formed,the symbol MD represents a intermediate area on which the second opticalpath difference providing structure is formed, and the symbol OTrepresents a peripheral area on which the third optical path differenceproviding structure is formed.

TABLE 2 Ring No. hs (mm) hl (mm) A0 A2 A4 A6 Pitch (mm) Central 10.00000 0.12527 0.00000 0.33265 0.02116 0.00000 0.12527 Area 2 0.125270.17728 0.00094 0.33275 0.02123 0.00000 0.05202 3 0.17728 0.217280.00188 0.33284 0.02131 0.00000 0.04000 4 0.21728 0.25108 0.002830.33293 0.02139 0.00000 0.03379 5 0.25108 0.28091 0.00377 0.333020.02148 0.00000 0.02983 6 0.28091 0.30793 0.00471 0.33310 0.021580.00000 0.02703 7 0.30793 0.33283 0.00566 0.33318 0.02168 0.000000.02490 8 0.33283 0.35606 0.00660 0.33325 0.02179 0.00000 0.02322 90.35606 0.37791 0.00609 0.33317 0.02188 0.00000 0.02185 10 0.377910.39861 0.00703 0.33324 0.02200 0.00000 0.02071 11 0.39861 0.418340.00798 0.33330 0.02212 0.00000 0.01973 12 0.41834 0.43723 0.008920.33336 0.02224 0.00000 0.01888 13 0.43723 0.45537 0.00987 0.333410.02237 0.00000 0.01814 14 0.45537 0.47286 0.01082 0.33345 0.022500.00000 0.01749 15 0.47286 0.48976 0.01177 0.33349 0.02264 0.000000.01690 16 0.48976 0.50613 0.01126 0.33338 0.02276 0.00000 0.01637 170.50613 0.52203 0.01221 0.33341 0.02290 0.00000 0.01589 18 0.522030.53748 0.01316 0.33343 0.02304 0.00000 0.01546 19 0.53748 0.552540.01412 0.33345 0.02319 0.00000 0.01506 20 0.55254 0.56722 0.015070.33346 0.02334 0.00000 0.01469 21 0.56722 0.58157 0.01603 0.333460.02350 0.00000 0.01434 22 0.58157 0.59559 0.01459 0.34150 0.000000.02274 0.01403 23 0.59559 0.60932 0.01545 0.34194 0.00000 0.021850.01373 24 0.60932 0.62278 0.01631 0.34238 0.00000 0.02104 0.01345 250.62278 0.63597 0.01716 0.34282 0.00000 0.02030 0.01319 26 0.635970.64892 0.01801 0.34326 0.00000 0.01962 0.01295 27 0.64892 0.661640.01886 0.34370 0.00000 0.01899 0.01272 28 0.66164 0.67414 0.018250.34399 0.00000 0.01839 0.01250 29 0.67414 0.68643 0.01908 0.344430.00000 0.01785 0.01229 30 0.68643 0.69853 0.01992 0.34887 0.000000.01735 0.01210

TABLE 3 Ring No. hs (mm) hl (mm) A0 A2 A4 A6 Pitch (mm) Central 310.69853 0.71045 0.02075 0.34531 0.00000 0.01689 0.01191 Area 32 0.710450.72218 0.02380 0.33275 0.02533 0.00000 0.01174 33 0.72218 0.733750.02479 0.33266 0.02552 0.00000 0.01157 34 0.73375 0.74516 0.024330.33243 0.02568 0.00000 0.01141 35 0.74516 0.75641 0.02532 0.332330.02588 0.00000 0.01125 36 0.75641 0.76751 0.09062 0.00000 0.59827−0.32850 0.01111 37 0.76751 0.77848 0.09347 0.00000 0.58204 −0.310020.01096 38 0.77848 0.78931 0.09633 0.00000 0.56669 −0.29303 0.01083 390.78931 0.80000 0.09771 0.00000 0.55189 −0.27724 0.01070 40 0.800000.81058 0.10058 0.00000 0.53808 −0.26276 0.01057 41 0.81058 0.821030.10346 0.00000 0.52496 −0.24936 0.01045 42 0.82103 0.83136 0.026740.34981 0.00000 0.01332 0.01034 43 0.83136 0.84158 0.02753 0.350250.00000 0.01310 0.01022 44 0.84158 0.85170 0.02686 0.35052 0.000000.01287 0.01011 45 0.85170 0.86171 0.02764 0.35096 0.00000 0.012670.01001 46 0.86171 0.87162 0.02842 0.35139 0.00000 0.01248 0.00991 470.87162 0.88143 0.02920 0.35182 0.00000 0.01231 0.00981 48 0.881430.89114 0.02997 0.35225 0.00000 0.01213 0.00972 49 0.89114 0.900770.02929 0.35251 0.00000 0.01196 0.00962 50 0.90077 0.91030 0.030050.35294 0.00000 0.01180 0.00954 51 0.91030 0.91975 0.03081 0.353360.00000 0.01166 0.00945 52 0.91975 0.92911 0.03157 0.35379 0.000000.01152 0.00936 53 0.92911 0.93840 0.03233 0.35421 0.00000 0.011380.00928 54 0.93840 0.94760 0.03164 0.35447 0.00000 0.01124 0.00920 550.94760 0.95673 0.03239 0.35488 0.00000 0.01112 0.00913 56 0.956730.96578 0.03314 0.35530 0.00000 0.01101 0.00905 57 0.96578 0.974760.03389 0.35571 0.00000 0.01090 0.00898 58 0.97476 0.98367 0.034630.35612 0.00000 0.01080 0.00891 59 0.98367 0.99251 0.03393 0.356360.00000 0.01068 0.00884 60 0.99251 1.00128 0.03467 0.35677 0.000000.01059 0.00877

TABLE 4 Ring No. hs (mm) hl (mm) A0 A2 A4 A6 Pitch (mm) Central 611.00128 1.00999 0.15581 0.00000 0.35317 −0.10591 0.00871 Area 62 1.009991.01863 0.15878 0.00000 0.34754 −0.10218 0.00864 63 1.01863 1.027210.16024 0.00000 0.34194 −0.09860 0.00858 64 1.02721 1.03573 0.163210.00000 0.33665 −0.09523 0.00852 65 1.03573 1.04419 0.16619 0.000000.33153 −0.09200 0.00846 66 1.04419 1.05259 0.16917 0.00000 0.32656−0.08893 0.00840 67 1.05259 1.06093 0.17065 0.00000 0.32160 −0.085960.00834 68 1.06093 1.06922 0.17365 0.00000 0.31692 −0.08315 0.00829 691.06922 1.07745 0.17665 0.00000 0.31236 −0.08046 0.00823 70 1.077451.08563 0.17966 0.00000 0.30794 −0.07789 0.00818 71 1.08563 1.093760.18116 0.00000 0.30350 −0.07539 0.00813 72 1.09376 1.10184 0.184180.00000 0.29930 −0.07302 0.00808 73 1.10184 1.10987 0.18721 0.000000.29522 −0.07074 0.00803 74 1.10987 1.11784 0.19025 0.00000 0.29123−0.06856 0.00797 75 1.11784 1.12577 0.19178 0.00000 0.28723 −0.066440.00793 76 1.12577 1.13366 0.19484 0.00000 0.28343 −0.06441 0.00789 771.13366 1.14149 0.19791 0.00000 0.27972 −0.06246 0.00783 78 1.141491.14928 0.20099 0.00000 0.27610 −0.06057 0.00779 79 1.14928 1.157030.20256 0.00000 0.27244 −0.05874 0.00775 80 1.15703 1.16473 0.205660.00000 0.26897 −0.05698 0.00770 81 1.16473 1.17239 0.20878 0.000000.26556 −0.05528 0.00766 82 1.17239 1.18000 0.21192 0.00000 0.26223−0.05364 0.00761

TABLE 5 Ring No. hs (mm) hl (mm) A0 A2 A4 A6 Pitch (mm) Intermediate 831.18000 1.18752 0.21381 0.00000 0.258730 −0.05205 0.00752 area 841.18752 1.19500 0.21670 0.00000 0.255927 −0.05070 0.00748 85 1.195001.20244 0.21961 0.00000 0.253166 −0.04938 0.00744 86 1.20244 1.209830.22255 0.00000 0.250447 −0.04810 0.00739 87 1.20983 1.21718 0.220050.00000 0.247375 −0.04682 0.00735 88 1.21718 1.22449 0.22302 0.000000.244742 −0.04561 0.00731 89 1.22449 1.23176 0.22601 0.00000 0.242144−0.04443 0.00727 90 1.23176 1.23899 0.22902 0.00000 0.239580 −0.043280.00723 91 1.23899 1.24617 0.23446 0.00000 0.237204 −0.04216 0.00718 921.24617 1.25332 0.23752 0.00000 0.234701 −0.04106 0.00715 93 1.253321.26043 0.24060 0.00000 0.232226 −0.03999 0.00711 94 1.26043 1.267490.24370 0.00000 0.229782 −0.03894 0.00706 95 1.26749 1.27452 0.241310.00000 0.227059 −0.03791 0.00703 96 1.27452 1.28152 0.24445 0.000000.224681 −0.03691 0.00700 97 1.28152 1.28847 0.24762 0.00000 0.227331−0.03594 0.00695 98 1.28847 1.29539 0.25080 0.00000 0.220009 −0.034980.00692 99 1.29539 1.30227 0.25645 0.00000 0.217833 −0.03404 0.00688 1001.30227 1.30912 0.25968 0.00000 0.215562 −0.03312 0.00685 101 1.309121.31593 0.26293 0.00000 0.213319 −0.03223 0.00681 102 1.31593 1.322710.26620 0.00000 0.211105 −0.03135 0.00678 103 1.32271 1.32945 0.263890.00000 0.208690 −0.03053 0.00674 104 1.32945 1.33616 0.26718 0.000000.206546 −0.02970 0.00671 105 1.33616 1.34283 0.27048 0.00000 0.204435−0.02889 0.00667 106 1.34283 1.34947 0.27379 0.00000 0.202359 −0.028100.00664 107 1.34947 1.35607 0.27958 0.00000 0.200410 −0.02732 0.00660108 1.35607 1.36265 0.28290 0.00000 0.198406 −0.02657 0.00658 1091.36265 1.36919 0.28622 0.00000 0.196443 −0.02584 0.00654 110 1.369191.37570 0.28953 0.00000 0.194525 −0.02514 0.00651 111 1.37570 1.382170.28719 0.00000 0.192460 −0.02451 0.00647 112 1.38217 1.38862 0.290470.00000 0.190641 −0.02385 0.00645 113 1.38862 1.39503 0.29372 0.000000.188876 −0.02322 0.00641 114 1.39503 1.40142 0.29694 0.00000 0.187168−0.02261 0.00639

TABLE 6 Ring No. hs (mm) hl (mm) A0 A2 A4 A6 Pitch (mm) Intermediate 1151.40142 1.40777 0.30260 0.00000 0.185613 −0.02202 0.00635 area 1161.40777 1.41409 0.30572 0.00000 0.184040 −0.02147 0.00632 117 1.414091.42038 0.30879 0.00000 0.182540 −0.02095 0.00629 118 1.42038 1.426650.31179 0.00000 0.181119 −0.02046 0.00627 119 1.42665 1.43288 0.309150.00000 0.179531 −0.02003 0.00623 120 1.43288 1.43908 0.31199 0.000000.178272 −0.01960 0.00620 121 1.43908 1.44525 0.31472 0.00000 0.177110−0.01920 0.00617 122 1.44525 1.45140 0.31734 0.00000 0.176051 −0.018850.00615 Peripheral 123 1.45140 1.48504 0.00356 0.43039 −0.028837 0.014810.03364 area 124 1.48504 1.52404 −0.02036 0.46504 −0.044431 0.017170.03900 125 1.52404 1.57735 −0.05909 0.51692 −0.066597 0.02036 0.05331126 1.57735 1.68994 −0.11461 0.58552 −0.093985 0.02403 0.11259 1271.68994 1.73016 0.04430 0.41718 −0.035205 0.01716 0.04022 128 1.730161.75573 0.35374 0.10533 0.068936 0.00555 0.02557 129 1.75573 1.775600.75304 −0.28449 0.195187 −0.00810 0.01987 130 1.77560 1.79219 0.445800.00000 0.106847 0.00103 0.01659 131 1.79219 1.80661 0.42668 0.000000.111826 −0.00002 0.01442 132 1.80661 1.81944 0.40049 0.00000 0.118620−0.00142 0.01283 133 1.81944 1.83107 0.36745 0.00000 0.127087 −0.003140.01163 134 1.83107 1.84173 0.32759 0.00000 0.137141 −0.00515 0.01066135 1.84173 1.85160 0.28093 0.00000 0.148717 −0.00743 0.00987 1361.85160 1.86080 0.22745 0.00000 0.161773 −0.00997 0.00920 137 1.860801.86945 0.16701 0.00000 0.176290 −0.01277 0.00865 138 1.86945 1.877600.09954 0.00000 0.192246 −0.01582 0.00815 139 1.87760 1.88533 0.024950.00000 0.209619 −0.01910 0.00773 140 1.88533 1.89268 −0.05690 0.000000.228411 −0.02262 0.00735 141 1.89268 1.89970 −0.14614 0.00000 0.248616−0.02638 0.00702 142 1.89970 1.90641 −0.24285 0.00000 0.270219 −0.030360.00671 143 1.90641 1.91285 −0.34713 0.00000 0.293211 −0.03457 0.00644144 1.91285 1.91904 −0.45913 0.00000 0.317596 −0.03900 0.00619 1451.91904 1.92500 −0.57889 0.00000 0.343357 −0.04365 0.00596

As shown in Table 1, Example 1 satisfies m1=0, m2=0, and m3=0. Further,d/f=2.67/2.2=1.21 holds. The value of d/f is relatively large and aworking distance for a CD is hardly secured in such the objective lensgenerally. However, in Example 1, the objective lens successfullyensures the working distance for CD, which is 0.359 mm. As can be seenfrom Tables 2 to 6, the minimum pitch of the first optical pathdifference providing structure is about 7.6 pa, and the average pitch isabout 14.39 μm. Therefore, p/fl=0.00345 holds.

It is further understood that well-balanced and high diffractionefficiency is exhibited for each type of optical disc, where thediffraction efficiency is 87.3% for BD, 74.6% for DVD and 60.9% for CD.

FIGS. 8 a to 8 c shows spherical aberration diagrams of Example 1. FIG.8 a is a spherical aberration diagram for BD. FIG. 8 b is a sphericalaberration diagram for DVD. FIG. 8 c is a spherical aberration diagramfor CD. In the aberration diagrams, symbol LM represents a sphericalaberration of main light which is a diffracted light flux with adiffraction order having the maximum light amount among diffracted lightfluxes with any other diffraction order, symbol 1 represents a sphericalaberration of unwanted light 1 which is a diffracted light flux inanother diffraction order having a less light amount than the main lightand is not used for recording and/or reproducing information for anoptical disc, symbol 2 represents a spherical aberration of unwantedlight 2 which is a diffracted light flux with another diffraction orderhaving a less light amount than the main light and is not used forrecording and/or reproducing information for an optical disc, and symbol3 represents a spherical aberration of unwanted light 3 which is adiffracted light flux with another diffraction order having a less lightamount than the main light and is not used for recording and/orreproducing information for an optical disc. As can be seen from FIGS. 8a to 8 c, excellent spherical aberration is kept within necessarynumerical aperture under the condition of m1=0, m2=0 and m3=0 for all ofa BD, DVD and CD, and recording and/or reproducing information can beconducted properly for the all types of optical disc. As can be seenfrom FIGS. 8 a to 8 c, phase shift is not caused at the boundary of thecentral area and the intermediate area, which is preferable.

Herein, chromatic aberration in Example 1 is 0.67 μm/nm. Table 7 showswavelength characteristics and temperature characteristics of Example 1.When the wavelength of the first light source changes by +5 nm, thethird order spherical aberration is −0.105 λrms, and the fifth orderspherical aberration is −0.024 λrms. When the temperature wavelengthrises by +30 degrees, the third order spherical aberration is 0.114λrms, and the fifth order spherical aberration is 0.025 λrms. These areconcocted by a magnification correction. Where, the magnificationcorrection means a correction of the magnification carried out bydisplacing the collimation lens.

TABLE 7 Before After magnification magnification Δλ + 5 nm correctioncorrection SA3 −0.105 0.000 SA5 −0.024 −0.006 SA7 −0.010 −0.006 BeforeAfter magnification magnification ΔT + 30 deg correction correction SA30.114 0.000 SA5 0.025 0.005 SA7 0.002 −0.002

Table 7 shows that the third order spherical aberration and fifth orderspherical aberration which are caused when a wavelength becomes longer,are both under-corrected (insufficient correction) in Example 1. Table 7further shows a preferable result that an absolute value of temperaturecharacteristics is small, which is preferable on the point of adisplacement amount of a collimation lens and of resolving power ofcorrection.

FIG. 9 shows a wavelength dependency of diffraction efficiency ofExample 1. As can be seen from FIG. 9, fluctuation of diffractionefficiency caused by wavelength fluctuation is controlled to be smallfor each of a BD, DVD and CD, and preferable results are obtained.

The present invention is not limited to the examples described in thespecification, and it is clear, for those having ordinary skill in theart in the present field, from the examples and spirits described in thepresent specification, that the invention includes other examples andvariations.

Objectives of the descriptions and the examples in the specification areillustrations persistently, and the scope of the invention is shown bythe claims described later.

1. An objective lens for use in an optical pickup apparatus whichcomprises a first light source for emitting a first light flux having afirst wavelength λ1, a second light source for emitting a second lightflux having a second wavelength λ2 (λ2>λ1), a third light source foremitting a third light flux having a third wavelength λ3 (λ3>λ2), andwhich records and/or reproduces information with the first light flux onan information recording surface of a first optical disc having aprotective substrate with a thickness t1, records and/or reproducesinformation with the second light flux on an information recordingsurface of a second optical disc having a protective substrate with athickness t2 (t1≦t2), and records and/or reproduces information with thethird light flux on an information recording surface of a third opticaldisc having a protective substrate with a thickness t3 (t2<t3), theobjective lens comprising: an optical surface including a central area,an intermediate area surrounding the central area, and a peripheral areasurrounding the intermediate area, wherein the central area includes afirst optical path difference providing structure and the intermediatearea includes a second optical path difference providing structure,wherein the objective lens converges the first light flux which passesthrough the central area, onto the information recording surface of thefirst optical disc so that information can be recorded and/or reproducedon the information recording surface of the first optical disc, theobjective lens converges the second light flux which passes through thecentral area, onto the information recording surface of the secondoptical disc so that information can be recorded and/or reproduced onthe information recording surface of the second optical disc, and theobjective lens converges the third light flux which passes through thecentral area, onto the information recording surface of the thirdoptical disc so that information can be recorded and/or reproduced onthe information recording surface of the third optical disc, wherein theobjective lens converges the first light flux which passes through theintermediate area, onto the information recording surface of the firstoptical disc so that information can be recorded and/or reproducedinformation on the information recording surface of the first opticaldisc, the objective lens converges the second light flux which passesthrough the intermediate area, onto the information recording surface ofthe second optical disc so that information can be record and/orreproduce information on the information recording surface of the secondoptical disc, and the objective lens does not converge the third lightflux which passes through the intermediate area, onto the informationrecording surface of the third optical disc so that information can berecorded and/or reproduced on the information recording surface of thethird optical disc, wherein the objective lens converges the first lightflux which passes through the peripheral area, onto the informationrecording surface of the first optical disc so that information can berecorded and/or reproduced on the information recording surface of thefirst optical disc, the objective lens does not converge the secondlight flux which passes through the peripheral area, onto theinformation recording surface of the second optical disc so thatinformation can be recorded and/or reproduced on the informationrecording surface of the second optical disc, and the objective lensdoes not converge the third light flux which passes through theperipheral area, onto the information recording surface of the thirdoptical disc so that information can be recorded and/or reproduced onthe information recording surface of the third optical disc, wherein thefirst optical path difference providing structure comprises a firstbasic structure and a second basic structure which are overlapped witheach other, the first basic structure is ablaze-type structure, whichemits a Xth-order diffracted light flux with a larger light amount thandiffracted light fluxes with any other diffraction order, when the firstlight flux passes through the first basic structure, which emits aYth-order diffracted light flux with a larger light amount thandiffracted light fluxes with any other diffraction order, when thesecond light flux passes through the first basic structure, and whichemits a Zth-order diffracted light flux with a larger light amount thandiffracted light fluxes with any other diffraction order, when the thirdlight flux passes through the first basic structure, a value of X is anodd integer, the second basic structure is a blaze-type structure whichemits a Lth-order diffracted light flux with a larger light amount thandiffracted light fluxes with any other diffraction order, when the firstlight flux passes through the second basic structure, which emits aMth-order diffracted light flux with a larger light amount thandiffracted light fluxes with any other diffraction order, when thesecond light flux passes through the second basic structure, and whichemits a Nth-order diffracted light flux with a larger light amount thandiffracted light fluxes with any other diffraction order, when the thirdlight flux passes through the second basic structure, a value of L is aneven integer, and the first basic structure has a paraxial power for thefirst light flux.
 2. The objective lens of claim 1, wherein the firstbasic structure exhibits under-corrected, aberration when wavelength ofan incident light flux becomes longer.
 3. The objective lens of claim 1,wherein at least a part of the first basic structure arranged around anoptical axis in the central area includes a step facing an oppositedirection to the optical axis, and at least apart of the second basicstructure arranged around the optical axis in the central area includesa step facing the optical axis.
 4. The objective lens of claim 1,wherein the value of L is an even number whose absolute value is 4 orless, and the value of X is an odd number whose absolute value is 5 orless.
 5. The objective lens of claim 1, wherein the objective lenssatisfies (X, Y, Z)=(−1, −1, −1) and (L, M, N)=(2, 1, 1).
 6. Theobjective lens of claim 1, wherein the first basic structure arranged inthe central area includes steps all of which face the opposite directionto the optical axis.
 7. The objective lens of claim 1, wherein thesecond basic structure arranged in the central area includes steps allof which face the optical axis.
 8. The objective lens of claim 1,wherein at least apart of the first optical path difference providingstructure arranged around the optical axis in the central area includesboth of a step facing an opposite direction to the optical axis and astep facing the optical axis, and the step facing an opposite directionto the optical axis and the step facing the optical axis satisfy thefollowing expressions (1) and (2), where d11 is an amount of the stepfacing the opposite direction to the optical axis, d12 is an amount ofthe step facing the optical axis, and n is a refractive index of theobjective lens at the wavelength0.6·(λ1/(n−1))<d11<1.5·(λ1/(n−1)),  (1)0.6·(λ1/(n−1))<d12<1.5·(2λ1/(n−1))  (2).
 9. The objective lens of claim8, wherein the first optical path difference providing structuresatisfies the expressions (1) and (2) in all over the central area. 10.The objective lens of claim 8, wherein the step facing the oppositedirection to the optical axis and the step facing the optical axissatisfy the following expressions (1′) and (2′):0.9·(λ1/(n−1))<d11<1.5·(λ1/(n−1)),  (1′)0.9·(λ1/(n−1))<d12<1.5·(λ1/(n−1))  (2′).
 11. The objective lens of claim10, wherein the first optical path difference providing structuresatisfies the expressions (1′) and (2′) in all over the central area.12. The objective lens of claim 1, wherein the number of steps facingthe opposite direction to the optical axis is larger than the number ofsteps facing the optical axis, in the central area.
 13. The objectivelens of claim 1, wherein, when any one of the first light flux, thesecond light flux and the third light flux whose wavelength is longerthan the corresponding wavelength among the first wavelength, the secondwavelength, and the third wavelength enters the objective lens, theobjective lens generates a third order spherical aberration and a fifthorder spherical aberration each of which is under-corrected.
 14. Theobjective lens of claim 1, wherein the objective lens satisfies1.0≦d/f≦1.5, where d is a thickness (mm) of the objective lens along theoptical axis and f is a focal length (mm) of the objective lens for thefirst light flux.
 15. The objective lens of claim 1, wherein theobjective lens generates a longitudinal chromatic aberration of 0.9μm/nm or less.
 16. The objective lens of claim 15, wherein thelongitudinal chromatic aberration is 0.4 μm/nm or more.
 17. Theobjective lens of claim 1, wherein the objective lens satisfies0.002≦p/fl≦0.004, where p is a minimum pitch in the first optical pathdifference providing structure in the central area and fl is a focallength of the objective lens at the first wavelength.
 18. The objectivelens of claim 1, wherein the first optical path difference providingstructure in the central area has a maximum pitch of 15 μm or less. 19.The objective lens of claim 1, wherein the objective lens satisfies−0.01<m1<0.01,−0.01<m2<0.01, and−0.01<m3<0.01, where m1 is a magnification of the objective lens whenthe first light flux enters the objective lens, m2 is a magnification ofthe objective lens when the second light flux enters the objective lens,and m3 is a magnification of the objective lens when the third lightflux enters the objective lens.
 20. An optical pickup apparatuscomprising: a first light source for emitting a first light flux havinga first wavelength λ1; a second light source for emitting a second lightflux having a second wavelength λ2 (λ2>λ1); a third light source foremitting a third light flux having a third wavelength λ3 (λ3>λ2); andthe objective lens of claim 1, wherein the optical pickup apparatusrecords and/or reproduces information with the first light flux on aninformation recording surface of a first optical disc having aprotective substrate with a thickness t1, records and/or reproducesinformation with the second light flux on an information recordingsurface of a second optical disc having a protective substrate with athickness t2 (t1≦t2), and records and/or reproduces information with thethird light flux on an information recording surface of a third opticaldisc having a protective substrate with a thickness t3 (t2<t3).
 21. Anoptical information recording and reproducing apparatus comprising theoptical pickup apparatus of claim 20.